To make for interesting souvenirs, I like to collect Hotel stationery/brochures or drink coasters from restaurants or cafes that I ate at.  Put these items next to the pictures of your hotel and of the restaurants in your scrapbook.   This will bring great memories of your trip to life!  

Try to learn what items are unique to the places you will be traveling and set aside some money to treat yourself to a little souvenir.  Venician glass, Italian gold in Florence, swiss army knife or swiss watch, etc.

Pictures - It takes some time out of your touring to take a lot of pictures.  You won't realize how valuable they are until months or even years after your trip.  Make an effort to snap as many pictures as you possibly can.  You will not regret it.

How about those ticket stubs, restaurant receipts, subway tickets, entrance tickets, etc. I even kept my notes about which trains to take and the stops to exit.  I figure they would go well with the pictures that I took while using the underground in London or the Metro in Paris.

Most important souvenir is to keep a written journal of your experience. Don't just write, "today we saw the Spanish Steps."  Wouldn't it be more interesting to write about what first came to your mind when you saw the Spanish steps.  For me, my first thought was watching Audrey Hepburn eating Gelato while sitting on the Spanish Steps in the classic movie, "Roman Holiday."  When you add your journal notes next to your picture, and the napkin of the Gelato that you purchased and ate while sitting on the Spanish Steps yourself, you will bring life to your scrapebook and picture album.  It will give you a very fond memory of your vacation.