Oh, wow, the list of fun things to do on a daytrip in Colorado is long and fun!
Colorado Springs and Canon City have a myriad of ideas. You can look most of them up online at http://www.pikes-peak.com/. I haven't done Seven Falls or the Royal Gorge yet but would recommend them anyway. There's Cave of the Winds, driving up Pikes Peak, Garden of the Gods (one of my favs), and more. Some are free. Some have a cost. Check the website for links to all of them.
Also, if you want to go west, the drive (US-285) to Fairplay and Buena Vista is quite scenic. There is an old museum from the prarie days in Fairplay. I think it's called Southpark. It's pretty cool because you can walk through different types of shops and see all the old tools of the trade. One building has old carriages of different sorts and there is a train car that you can climb into.
Or, you can go out I-70 to Mt. Evans, Glenwood Springs, Idaho Springs, Historic Georgetown, etc.
If you like to hike, try localhikes.com for some good locations with easy trails where you can take the kids.
Pine Valley Open Space in Jefferson County has a big lake & a river where you can go fishing and/or feed the ducks or take a hike.
If you want to go north, you can try a drive through Estes Park or a walk through the old part of Ft. Collins or Longmont.