Golden Gate Canyon State Park

Golden Gate Canyon State Park Panorama Point Scenic Overlook

Golden Gate Canyon State Park is only 30 miles from Denver, but it encompasses 12,000 acres at an elevation range from 7600 feet to 10,400 feet.  The state park has 36 miles of hiking trails and has 22 miles of trails for mountain bikes and horseback riders to enjoy.
Camping is the reason at Golden Gate Canyon State Park, and the campsites are highly rated.  Do plan ahead and book your site before traveling.  Backpacking sites usually don't require reservations, just avoid holidays and camp during weekdays if you can.  For questions about Golden Gate Canyon, call the park at 303-582-3707.
The park hosts 134 campsites plus backcountry sites that are not too far away from a road.  They even have a few shelters at the backcountry sites, built as lean-to's like used on the Appalachian Trail. There are cabins and yurts at $60/night/6 people, plus the historical Harmsen Ranch Guesthouse at $220/night/8 people for overnight lodging.  The camping areas also handle tents at $14/night (no electric) and trailers and RV's $18/night, some with electrical hookups.
For a tent-only experience, plan to camp at Aspen Meadows.  Tents, trailers and RV's are welcome at Reverend's Ridge in the park.  No tent, no problem.  Just book a cabin or yurt to spend the night.
By the way, the owners of the Harmsen Ranch Guesthouse were the founders of Jolly Rancher candy company in Denver.  Sweet!  

View within the park

The park features snow capped mountain views, sub-alpine wildflower meadows, pine and aspen forests and offers fishing along with hiking and rock climbing.  Hunting in Jefferson County is allowed during the season.  The Panorama Point Scenic Overlook offers views of over 100 miles of snow packed peaks along the continental divide.
While the park is generally busy year-round, you can still keep to yourself on the backcountry trails and campsites.  This is an excellent location to test your new backpacking equipment or train for your long backpacking trip as your car is never more that a few miles away.  You still get the cool nights, wildlife and outdoors peace at significant elevation.
The Park Map will give you a good idea of what to expect.
Keep in mind it is close to the metro Denver area for a quick weekend or weekday get away and you won't be the only person who had that same thought.
To get there, take highway 93 north from Golden, Colorado for a mile and a half, then turn left onto Golden Gate Canyon Road and follow it for 13 miles to the park entrance.