There are free things you can do at both Disney and Universal, although they are designed to lure you in to spend money. For example, Disney has both the Boardwalk and Downtown Disney. And if you didn't want to spend the full price on Disney and wanted to go "Disney-lite" you might want to check out Disney Quest. Disney also has water parks that might fit in the budget more.
If the boys are typical boys that like things that go boom, Cape Canaveral is about an hour away. An hour up the road from there is Daytona Beach, if they like things that go fast. Not just the track, but at one time there was a Corvette museum up there also.
As of 2009, there was a flight museum of some sort about halfway between Orlando and Tampa on I-4. Fantasy of Flight I think is the name, but having never been there, I can't vouch for the quality you might get there.
Really, Orlando is the type of place that once you decide what you want, you'll find it.