Souvenirs get expensive actually. I used to bring the kids things, but they already own so many things. New things that were actually in my budget (and I go away at least 4 times a year) just ended up in already existing piles of things, forgotten minutes after they were received. Postcards, though…postcards I approve of. You can get lot of postcards for a little bit of money (it’s a good idea to travel with your own stamps, though: touristy places are usually out, and it’s not always easy to get to a post office) and it’s always nice to get real mail, especially when you’re a kid and no one sends you mail.
I have pretty much every postcard anyone’s ever sent me, which is a decent number of postcards, but, being mere scraps of paper, they still fit in a single milk crate, with room left over for another couple of decades of postcards. I imagine that I’ll want them toward the end of my life. Every once in a while I dig through them for a bit, but mostly it’s nice to just have a box of tangible proof that people think of you from time to time.