Dillon Lake above and the Loveland Pass above the Eisenhower tunnel on I-70 below.

Below: Rocky Mountain National Park located just west of the village of Estes Park, Colorado, (about 2 hours from Denver) is a day-hiker's paradise with hundreds of well kept trails for all abilities accessing summits and lakes. Plus, the Trail Ridge Road over the top of the park offers views nowhere else available (highest through road in the US). Estes and the national park offer one of the great places in the US to view wildlife.

Otis, Hallets and Flattop Mtns from near Bear Lake

Longs Peak from the park entrance. Bull elk - Rocky Mtn Natl
Park and Estes Park are a fantastic areas to view elk all year.

Garden of the Gods and Pikes Peak (l) are
less than two hours south of Denver on I-25. Mt. Evans (r) is just west of Denver.
Garden of the Gods has many short walking trails through the many amazing
structures. You can drive to the top of Pikes Peak or take the day trip on
the cog railway to the top. You can also drive near the top of Mt. Evans.

Indian Peaks Wilderness (l) is just northwest
of Denver not far from the city of Boulder. Arrowhead Golf Course (r)
is an area
highlight.
Plus, there are many mountain towns like Estes Park, Telluride, Steamboat Springs, Breckenridge, Aspen, ... all of which offer great vacation options. Steamboat Springs has a pro rodeo every Friday and Saturday evening throughout the summer. Plus you can rent a 4-wheeler for off road fun in Big Red Park north of Steamboat.
Four-wheeling on Rabbit Ears Pass near Steamboat Springs
VISIT A NATIONAL PARK: from Rocky Mountain to Dinosaur to Mesa Verde
Fishing? There is great fishing in Steamboat Springs on the Yampa or Aspen on the Frying Pan. Guide services and equipment rental are available.
Here's some fishing links: Colorado Trout Hunters,
Check out Grand County for lots of activities (within 3 hours of Denver) on the west side of Rocky Mountain National Park.
Float trips? The Arkansas River along Highway 50 west of Canyon City has many rafting companies. Also Clear Creak along I-70 near Idaho Springs just a few minutes west of Denver offers rafting. Both are within easy driving distance from Denver.
Rafting & Float trips click
HERE
for southwest colorado
or HERE for a variety of activities in the Breckenridge/Keystone area less than two hours from Denver
or HERE
for Canyon City and other locations throughout Colorado