I'm a kid myself, so I've never travelled w/ kids. I'm not sure how helpful my suggestions are, but here goes.

I've taken day trips to Chicago, SFO, SEA, MCO, EYW, DEN, EZE.

Chicago was nice, saw some museums and walked around downtown. It has a nice lakefront and a pretty good amount to do.
SEA has the Boeing museum, I also hope to get to everett within a month or so.
In MCO, I didn't touch Disney. That'd take more than a day.
EYW was awesome to leave the Pennsylvania snow and ride a moped on a 2x4 mile island, probably even nicer by comparison leaving from MSP. Theres not too much beach, I didn't swim at all, but it's really neat to explore old town.
DEN is a nice city with a beautiful downtown. LoDo has the 16th street mall, some nice landmarks, a six flags, and ....The COLORADO AVALANCHE(ColoAvs).
EZE is a little long for a day trip.

SEA, SFO, DEN and the west coast are nice because you have the entire day, you just take a red-eye home and are tired the next day.
The rest of the country you have less time at your destination, but you get to sleep in your own bed instead of a plane.

I've enjoyed every place I've been, and love to do day trips. They just run you a little ragged by the time you get home.