I'm a big souvenir fan, and we get a lot every time we go. T-shirts for the kids, to remember that we went, a photo album, picture frames, the kids will spend some of their money in the big Toontown store near Mickey's house, things like that. Some of the big things we have got in the last couple years are: the pictures you can make in the store after Star Tours, where they can superimpose your picture in a Star Wars scene. DS is a huge Star Wars fan, so that was great (expensive, but priceless)! The first time we went, we had the artists in the Main Street shops draw something the kids wanted that you choose out of their books (DS got Mickey, DD got Cinderella), and had the artist personalize it for the kids, with names, dates, etc... This was something really special that we had framed and they can always remember the first time they went.
I also get a watch every year we go. In 2003, I got a really nice one from the jeweler on Main Street, it was a limited edition Fossil watch with a blue face, the 4 icons of WDW and a big silver "2003" in the face. Looks really nice. Last year, I got a gold Citizen watch, with a gold Mickey mouse lightly imprinted on the face. Also very nice. Expensive items, but you can only get them on Main Street, especially the limited editions.