A lot of good suggestions have already been made, but I haven't seen the Littleton Historical Museum mentioned yet. It's like taking a little trip back in time, there are two farms, one 1860's and one 1890's, if I recall correctly. You can see the animals, a blacksmith shop, schoolhouse, houses from those time periods, complete with the old-fashioned stoves and bathtubs. There are people in period dress who explain how things used to be done in the old days. The kids always love it, and it's actually educational, too!
http://www.littletongov.org/museum/events/default.asp
I would second the Corn Maze, too. My kids love it. Both of these attractions are in the Littleton area.
Some other places listed (Pike's Peak, Air Force Academy, Cheyenne Mountain Zoo, Royal Gorge and Seven Falls) are down in or beyond Colorado Springs (1.5 hours from Littleton), so be sure you know where a place is before you get too hooked on the idea. It might be a bit much to drive down there more than once, but we'll often do one long day trip down and visit several attractions. Like drive down early, visit Pike's Peak or Cheyenne Mtn Zoo in the morning, then Garden of the Gods or Cave of the Winds in the afternoon.
And yes, with a six year old -- you HAVE to have one meal at Casa Bonita. The food is awful, but the cliff divers make it worth it.
