Ismac - check out the science center - there is an agreement amongst most science centers in the country (and I know that Orlando participates in this, as their previous director is in my city now) that if you have a membership to another science center somewhere, you can use that for free entry into any other science center.
This would be a rather inexpensive excursion, even if you had to buy a science center membership - ours cost $35.00 per year for a family membership --
Another suggestion, and one that our family always does when we are in Orlando, is to resort hop at WDW - especially if you will be there during the holiday season to see all of the decorations, etc. You can park at DTD and take a bus - plan your route so that you can take one of the boats in the MK area, monorail, etc. A great freebie trip that my girls always enjoy!!
Have fun!