I think that if they are really not interested in the rides, then either Epcot or Sea World are the best places to visit. That said, is it set in stone that they are 'too old' for the rides? I think your dad is right about Magic Kingdom - there is loads of entertainment there and the rides are really fun without being too extreme.
My husband has recently hit 40 and there is no way we would be satisfied with the tame and boring parks and can't envisage it in the future either. Universal IOA is our favourite.
They may also consider visiting Kennedy Space Center instead of the parks on one day. It's a half day excursion at most, but maybe at their age they'd enjoy it?