If you get an autograph book, try to get one with a plastic sleeve where you can insert a picture of the kids with the character right above the signature. These are really neat. Also, you can purchase the EPCOT passports at just about any of the World Showcase shops for about $10.
There are also some good shops at the Boardwalk that you can check out as well. If you have daughters, they might be interested in getting a special hair braid (consisting of beads and colored thread). My two daughters have gotten them everytime they've been to WDW and I know Boardwalk has a place that does the braids. They stay in the hair for about 4-8 week and typically cost $25 each. Pricey but they do look pretty cool.
Finally, you could get your kids into pin collecting. Be prepared though, you could spend a mint on trying to collect the numerous types of pins. In fact, there's a separate discussion board dealing with Disney collectibles, including pins.
Good Luck!