You should photograph whatever you want, because this is the best way of ensuring shots which are imaginative and creative.   However, it doesn't hurt to think about the different types of things you could photograph, in order to whet your appetite.   Since you're doing travel photography, it makes a lot of sense to concentrate on the things which are unique or distinctive about the place you've spent so much money to reach - there's not a lot of point photographing your friends at a hotel or golf course, if you could have been getting identical shots back home! 

    One of the most obvious things to photograph when travelling are the landscapes which are unique to the place, or which make it world famous, like Ayer's Rock in Australia or Niagara Falls in the USA.   They've been photographed millions of times, but don't feel as if that should stop you from making at least a few more hackneyed photos.   And if you can find some way to lift the shot out of the ordinary, all the better.   Consider whether you want these photographs to include people or not.   Sometimes having a person in the shot is important to give a sense of scale, and sometimes it's better to leave people out of the picture.   The famous American landscape photographer Ansel Adams was considered radical in his day for leaving people out of his landscapes, at a time when society thought in terms of the "taming" of wilderness.   His photos returned a feeling of grandeur and power to the countryside.   However, if you do decide to put people in the shot, then why not photograph your travelling companions and yourself, as well, standing in front of those unique landscapes.   If you do take this sort of photograph, then you should realize that it's very difficult to get everything in sharp focus, both yourself and the thing behind you, especially if it's a mountain or something far away.   The best way to get things in focus is to put the camera on a tripod and shut the aperture as far down as it'll go, to f22 or f32.   You'll need to use a slower shutter speed than a handheld photo, so you'll have to make sure you hold still during the exposure.

    Another subject area is the people in the place where you're travelling, especially if their clothing or other customs are different to what you're used to.   Again, these sorts of photographs have all been done before, but it's your vacation and it would be crazy not to do them at least once more!

    Local architecture can be a very fruitful area for photography.   Sometimes this is a no-brainer, like the Eiffel Tower in Paris or the Taj Mahal in India.   Religious architecture can be very interesting, whether it's Christian churches or cathedrals, Muslim mosques, Buddhist temples or whatever.

    Another aspect of the local culture which might not have occurred to you as something to photograph are signs - road signs, store signs, anything that strikes you as interesting or unusual.   You'll find interesting and amusing signs anywhere you travel, in your own country, overseas and in places which don't even speak English.   As examples, take a look at these Trinidadian signs and Japanese signs.

    One underexploited resource is the wildlife which is peculiar to a particular place.   The whole notion of ecotourism is a fairly recent development, and most ecotourists scarcely begin to scratch the surface of the extraordinary wildlife of the places they visit.   Some visitors do make an effort to see large mammals or birds, but very few actively seek out reptiles or insects, even though these can be extremely colorful and interesting.   Unfortunately, it's not always easy to photograph some of these animals, not only because it requires specialized camera equipment like telephoto and macro lenses, but also because it takes a significant amount of patience and skill.
 

taking photos from moving vehicles

    Travel normally involves some amount of travelling, which generally means moving vehicles!   You might also decide to take a boat or aircraft tour while travelling.   In either case, these situations can be very valuable from a photographic point of view, but you need to make some adjustments to your normal techniques to get good results.   The main issues arise from vibration of the vehicle, and reflections and distortions from any windows between you and the thing you're photographing.

    If you're taking a photo from a moving vehicle, or even from a stationary vehicle whose engine is still running, then you need to take vibration into account.   The normal rule of thumb is to set the shutter speed to the inverse of the focal length of the lens, or faster - for example, for a 100mm lens you'd set the shutter speed to 1/100th of a second, or faster.   However, if you're in a moving or vibrating vehicle, then you should double or even quadruple the shutter speed, meaning a speed of 1/200th or even 1/400th of a second for a 100mm lens.   If you don't do this then you might end up with blurry photos.

    The other main problem with moving vehicle photography is reflections from the vehicle's windows.   The primary solution to this problem is to move the camera as close as possible to the window; but if the front of the lens is in contact with the glass then it will directly transmit the vehicle's vibration to the camera and make it even more difficult than normal to get a sharp picture.   If you have an SLR camera then one way around this is to put a rubber lens hood onto the front of the lens, and hold this hood against the glass while keeping the actual metal or plastic barrel of the lens off the glass.   If you wear very dark clothing then this will also reduce the amount of reflection.

    Another problem is distortion caused by taking photos through curved glass.   If you're taking photos from an aircraft, then this is a particular issue if you're sitting in the front seat next to the pilot of a small aircraft, or if you're in a helicopter, especially in the front seat.   Always check the curvature of the glass before you take the photos, and avoid taking photos through the most curved sections.   It's often not easy to see the distortion when you're looking through the viewfinder, but it's usually obvious in the final result, which is why you should check beforehand, by looking through the glass without the camera while moving your head around to cover different angles.   Other people might think you're odd, but you will get better shots!
    If you're in the front seat of a single engined plane, then you'll also have to deal with the propeller getting in the way.   If you're using a digital camera then you can afford to take more shots than usual and delete or crop the ones which include the propeller, otherwise you'll just have to take your chances.   Increasing the shutter speed will reduce the area of the shot affected by the propeller; reducing the shutter speed will reduce the effect of the propeller by spreading it over a wider area, but then you run the risk of increased camera shake.   Often you'll be better off sitting in the seat nearest the back of the aircraft, where you'll have a view which is less obstructed by the wings, and of course the propeller won't get in the way.    If you have the choice of aircraft type, then a plane with high wings will usually have fewer obstructions than aircraft with low wings (ie, with the wings below cabin level). 
    Many vehicles such as airliners use polarized glass in their windows.   If you put a polarizing filter onto the front of your lens, then the two polarizing surfaces interact with each other, creating rainbow colored patterns in bands across the photo.   This effect might sound intriguing and appealing, but in practice the interaction almost never produces a desirable effect - unlike flare, which can sometimes be used deliberately to add appeal to a photo.  If you're not certain whether the window is polarized or not, you can put the filter onto the lens then look through the viewfinder while moving the lens up and down, and from side to side; normally, the interaction is fairly visible.   The only solution is not to use a polarizing filter on the lens.
    If you're flying in a jet or turboprop aircraft, then try to get a window seat in front of the wing, since the hot jet exhaust causes severe distortion.   Turboprops might look like innocent propeller driven planes, but those propellers are actually powered by jet engines, which is why this same warning applies to them.   You can usually recognize a turboprop by the presence of large round pipes coming out the back of the engine.

    On a boat trip, the front of the boat will usually have the best views, but of course you need to watch out for spray - most cameras don't like water too much!   If you have problems with seasickness, then stay outside the cabin in the fresh air, stay as far back in the boat as possible, since the stern moves up and down less than the rest of the boat, and concentrate on the horizon, rather than looking down at the water, or the boat itself.