I lived there or 6 years. Ft Collins is cool. A must see is the New Belgium Brewery. Best beers in the country. Great downtown area and there is the Poudre canyon which is a fun drive. The road GOes all the way to Steamboat
Buena Vista is a great camping area. Excellent rafting close by on the Arkansas river. The rafts take you thru a 1200 ft deep canyon. Very fun.
Colorado Springs is ok. There is a place called Garden of the Gods which is a very cool rock formation. Pikes Peak is there and you can drive to the top. Rodeo Hall of Fame (never been) and the Air Force Academy.
Vail is great. Lived there for years. PLenty of good restaurants. COntinental divide. Great camping. Rafting, Fly fishing, you can mtn bike or take the bikes up the chairlifts and ride them back down the mtn.
Proffitt;s Cruisers is in delta, CO. Middle of nowhere and not much around it but if you're a cruiser nut....
Boulder is beautiful. A bit too hippie for me but there is a great feel to the place. Great breweries and restaurants. Mtn's close by. Definitely worth a day trip.
Lived in Denver for 5 years. Plenty of fun stuff. Stay at a hotel near the 16th street mall. Nice area and walking distance to everything. Very safe, clean and fun city
Check the concert schedule at Red Rocks Amphitheater. You can go there during the day too.
Blackhawk and Central City have casinos. Less than an hour from Denver. Old mining towns that they turned into a mini-vegas