We did MK in 2001 on Veteran's Day, and it was a little crowded (this was right after 9/11, so all of WDW was slow), but not too bad.
How about planning on a water park? Or going to DisneyQuest right when it opens. Depending on the weather, this is what I'm planning for the Monday of MLK weekend. Or schedule a character breakfast at a resort and really take the time to enjoy it!
SeaWorld and Universal will be just as crowded as MK, I'm guessing. Lots of people go here over long weekends also!