Hi Lisa,
If you have extra time, I'd say that the Disney water parks are a must do. My family prefers Blizzard Beach because of the dedicated area for elementary school age kids (like yours) but Typhoon Lagoon is great also.
Of course you'll want to take in some shopping at Downtown Disney. The Cirque du Soleil show at Downtown Disney is AMAZING.
There is great recreation at the resorts. May have boats and bikes to rent, which is a great way to get some non-walking activity into your vacation.
I love the the Hoop Dee Doo Revue dinner show at Fort Wilderness. I started bringing my kids when they were four years old and we've been back every year since.
Please also check out the tips offered in our "Mom Views" area where we ALL get to give advice to guests. This question about
off the beaten path ideas seems totally up your alley.
Enjoy your visit to Walt Disney World!