We are flying to Denver in August for a convention. We would like to extend our trip for a week. We will have 4 adults and two little ones, 4 and 1 and are trying to think of something that would be enjoyable in light of the little ones. Thought about houseboats as the men like to fish. Any ideas would be so appreciated.

Houseboats are probably not feasible from Denver.

But you could fish at Grand Lake, which is a beautiful natural mountain lake on the backside of Rocky Mountain Nat'l Park. It connects to Lake Granby and Shadow Mountain Lake (I think those are the other two names of lakes). Lots of marinas to rent decent speedboats.

Drive from Denver to Estes, spend a night or two in the park, go to Grand Lake and spend another night or two. Drive home over Berthoud Pass for gorgeous scenery.

PS. If you want fly-fishing, there are good choices on that route, too.

Go to Idaho Springs and ride the railroad. Fly fishing--yes. go all the way to Durango and do the railroad trip.
Denver has a great zoo and museums your little ones would enjoy. Spend some time there.

The railroad is in Georgetown and and the four year old would love it. No houseboats in Colorado. No deep water other than a couple of lakes. There is an Alpine slide in Breckenridge. Tiny town in the foothills west of Denver would be a hit for the four year old. The Breckenridge, Frisco area would provide enough entertainment for everyone.

OH, wait. Go to Morrison and the little dinosaur museum there, or to woodland Park, outside of Colorado Springs, for the really really good dinosaur museum there.
Maybe Manitou Springs--Denver folk. Not MY favorite place, but maybe something there for these.

All in the greater Colorado Springs area:

Florissant Fossil Beds National Monument (www.nps.gov/flfo/), plus lots of fun tourist stuff to do (Cave of the Winds, Garden of the Gods rock formations park, Cheyenne Mt. Zoo, Manitou Springs ancient cliff dwellings) too many and varied to list.