Most fun domestic trip we've done was San Diego for a few days. Specifically
Mission Bay where we played in the pool, beach, SeaWorld, zoo, etc.. After a while we drove from SD to Scottsdale and explored everything AZ has to offer. While there we stayed in a nice resort with an awesome kids club and several pools. Fairmont Princess. I highly recommend something similar to that.
We've done cruises, beach vacations, etc...but this trip was by far the best for our little kids.