Most of it depends on the ages of your children. I would stay away from getting/giving too many things and focus on experiences, since that's what they won't get again. Getting the Memory Maker photos is a great idea. Some of the photographers are great and some are so-so, but we've always come home with enough amazing photos that make it worth it. You still need your own camera, but it's easier to get whole family photos this way.
Here's my list of (free) ideas, some of which are a little outside of the usual thoughts, which may or may not be of any interest to you, depending on what your children are interested in:
At DHS, there's a free drawing class at the Animation Academy. Every half hour, I think.
At Epcot, try the Agent P missions in World Showcase (there are "registration" booths) and the Kidcot stops in each land where you get a bear signed by representatives of the country. Five years ago, if you took the finished project to guest services near Spaceship Earth, you got a special prize (some trading cards and a coloring book). Stop in Club Cool at Epcot and try different sodas from around the world. Also, check the schedule for the Jammitors -- a group of janitors who play percussion on garbage cans -- very cool!
At AK, make sure to visit the tree roots up close to look at all of the animal carvings. They just opened some new ones recently. Very few people go look so it's uncrowded on those trails. There's also a wildlife badge earning program with different stations for kids there too -- can't remember what it's called because my boys haven't tried it. They did the previous version and enjoyed it, so I'm sure this one is good too.
At MK, play Sorcerers of the Magic Kingdom -- get your first set of cards and instructions at the firehouse, but after that, pick up your daily set of cards in Liberty Square because it's never crowded at that place. It's a little hidden though. After you ride the speedway in Tomorrowland, you can ask if they have any driver's licenses for you -- sometimes they do and sometimes they don't, but they're cute little souvenirs. Also in Tomorrowland, watch out for PUSH, the rolling trash can -- he's pretty funny! Make sure you see the opening show at MK one morning, about 15 minutes before the park opens.
Finally, on most of the modes of transportation (bus, some boats, and monorail) the drivers sometimes have transportation trading cards that the kids can ask for. My boys have been trying to collect the entire set for years so it's a nice challenge and another free souvenir. We've learned that there are different kind of buses at Disney and lots of other (useless, but good for entertainment during a bus ride) facts!
These are the little things that make our trips magical, more than the big splashy, expensive things. Have a great trip!
Edited to add: I almost forgot -- arrange for a wake up call one morning (preferably for a time after you know everyone will already be awake) and let the kids answer. We haven't done this in awhile, but if I remember correctly, Mickey calls and Stitch interrupts him.
