The denver children's museum is a great day out. Your child is bound to enjoy the story times, the firetruck and the grocery store! Whether your kids are babies or 10, there is something there for them.
After a few hours I am exhausted, but my daughter is still going strong. There is just enough to do that you can hit it all in a day, but not too much to where you feel overwhelmed.
Take a day, drive downtown, and let your kids run wild! They will take a great nap on the way home!