1) Rocky Mountain National Park
From Denver, take the SR 36 all the way through Boulder into Estes Park. In Estes Park, load up on supplies before you head through the official main gate ($20) of the Rocky Mountain National Park. First road on the left after the main gate is Bear Lake Road. Follow this road until the end, and stop to take pictures or enjoy the view where there is a opening on the side of the road or a sign for a landmark (lakes). At the end of the road, you will see the beautiful Bear Lake. From Denver to Estes Park is 2 hours. Trip inside the park is 45 minutes.
2) Invesco Field at Mile High Stadium Tour
and call the box office/team store to ask about tour availability. It wasn't available this week but it may be when you visit. I'm sure it will be a fantastic experience.
3) Capitol Building, City Sunset Skyline
This is in the heart of downtown. I suggest (if you have a rental car) you take a drive around the state capitol, the U.S. mint, and the city's beautiful art museum during sunset, and you may be able to catch a glimpse of the downtown skyline. Capitol building accessible by I-25, Colfax Ave. exit.
4) Writers & Larimer Square, 17th Street Mall
Heart of downtown, find on downtown map (available at hotel). Dining (Palomito on 17th Street Pedestrian Mall is my choice), bars, and fun. The squares and the mall are relatively safe like the rest of the downtown area.
5) Invesco Field Game Day
Get there 2 hours early. If you're in a hurry to leave or get on the freeway afterwards, park higher up on the hill in the private lots, (you will see it when you get there) as those lots are closer to the I-25 on-ramp. Take the Mile High Walk around the stadium, visit the Denver "Hall of Fame", and get to your seat early to watch the players run out onto the field and practice.
TIP: The speed limit in Denver is average 40-60 MPH, much slower than other states. Also, there are many cops and traffic lights are camera enforced. We got nabbed TWICE.