Passport stamps: He dies dies with the most stamps, wins!
Thats my travel philosophy, anyway.
I always go for keychains, bottle openers, and magnets. You can get them for like 6 for $5 (or cheaper). Sure your friends have bottle openers, but do they have one from Europe? Keychains they´ll use daily. Megnets, no surprise there. Everyone uses kitchen magnets :D
Another cool and free souvenir is: everytime you go into a pub, grab as many different kinds of coasters from different countries as you can. They´re cool, they´re unique, and every brand of beer has one. It´ll be a nice little international collection that you can start, and will cost you nothing.
Throw them in your journal and buy a cheap photo album to put them in when you get home