What do you want to see? Everyone has different interests. I don't think there is much of anything that is universally "must see" except for some uniquely preserved ancient city sites or rare geologic features, and even so, you have to be interested in those things.
For camping and fishing I recommend
Curecanti National Recreation Area. This has 10 campgrounds to choose among. You'll need a boat for access to the best fishing areas, I'm pretty sure they can be rented locally.
My kids like the area around
Rocky Mountain National Park. An abundance of recreational opportunities and easy access from urban Colorado make this one of the busiest parks in the system.
My favorites are where I can see the geology.
Black Canyon on the upper Gunnison River,
Royal Gorge on the upper Arkansas River, and
Colorado National Monument where the Gunnison joins the Colorado and starts cutting into the Colorado Plateau to create canyon country (which is mostly in southern Utah and northern Arizona.
Interested in pre-Columbian culture? Then
Mesa Verde in southwestern Colorado, although you will find more of these settlements more or less along the Rio Grande in central and northeast New Mexico.
Old mining towns in a beautiful mountain setting, with lots of resort visitors and active sports activies?
Durango in southwestern Colorado.
Do you want to soak in hot springs? Colorado has a
couple dozen locations, my preference is
Steamboat Springs, a major winter resort that stays active through the summer.
Other people visit the cities along the Front Range. Pueblo, Colorado Springs, Denver, Boulder, Fort Collins each offer a different flavor of urban life with the Rocky Mountains in the background. I like the atmosphere in Boulder, some of my friends keep going back to Colorado Springs in the shadow of Pike's Peak.
