Let me ask you a question. Do you find yourself drawn to something specific when you are traveling? No matter where you go...you see it and are drawn to it? This could be anything from as broad as architecture to something as specific as door knockers. Well, let me lay an idea on you. Why not create a photo collection of this specific "thing" that you are obsessed with whenever you travel? It is just like collecting stamps or baseball cards...your are just doing it with your camera. Whenever you see that certain "thing" take the best photo possible given the situation and soon you will have a growing collection of photos that you can create into an art piece, share on a website or just keep it a secret for your own enjoyment.
One of the main keys to creating this collection is staying consistent. Figuring out what the object of your attention will drive your whole collection. That is not to say that you can't have multiple collections going on at the same time. There is no rule against that.
Let me give you an example of my photo collections that I am always cultivating. Two of my photo collections are windows and doors. I don't know why, but it seems that when I travel the windows and doors I see are always more interesting than what I see every day. I find there is a lot of character and beauty in them. Plus, I find a lot of symbolism in both. I am also obsessed with getting photos while I am flying. Cloud formations and photos of the earth below always fascinate me.
Now you can figure out what it is that you gravitate towards...take out your camera and start shooting. Here are some other examples of subject matter for photo collections that I have seen or heard of. I would also love to hear some of your ideas or what you love to collect.
cracking or weathered paint
knots in wood
empty swimming pools
You put one photo of a window out there and it doesn't really mean all that much, but when you put 50 or 100 images of a window out there it becomes a collection. Here are a few more from my collection.