The print above was purchased at the top of the Eiffel Tower, although it's just a mass produced print it reminds me of the moment we were there.
This one is just a locally printed greeting card sold in a street booth, but again, it holds the memory of our trip.
Photography is also fun to buy. It's often reasonably priced, and if the artist is the one selling it don't be afraid to get it signed. (The little old man who took these was happy to sign them for me.) It's just a small detail, but I think it adds a bit of charm and authenticity to the piece.
The things you hang on your walls should make you happy, they should remind you of the people, places, and things you love, and they shouldn't have to break the bank.