I live in the Denver area, and if you want to concentrate on the outdoorsy things you mentioned, I wouldn't recommend using Denver as a home base. Pike's Peak and Garden of the Gods are in Manitou Springs (close to Colorado Springs), and the Royal Gorge is another maybe 30-60 minutes south of there. So, while you can do day trips to those places from Denver, you'd spend much less time on the road if you stay in Colorado Springs. In the Pike's Peak area is also Cave of the Winds, where you can take a regular cave tour with lighted walkways and stairs or a more adventurous tour crawling around on your belly with headlamps on your hard hat. (reserve the adventurous one ahead of time.)
The pro rodeo hall of fame is also in Colorado Springs, if that interests you, and there's a very nice little zoo called Cheyenne Mountain Zoo, where you can feed the giraffes.
I'd also recommend staying a few days in the mountains. Again, you can make day trips there from Denver, but it's simpler to stay there. We like Glenwood Springs. They have a huge hot-spring fed pool with a couple of water slides. There's an "adventure park" with a gondola ride up the mountain, and at the top, an alpine coaster, a giant swing that swings out over the canyon
and another cave tour. Rafting is nearby as well.
Ski areas can be a blast in summer, too. Mountain bike down the trails, some have alpine slides, and stuff like mini golf, climbing walls, bungy trampoline, etc in the base area, and again, you could probably find rafting relatively close to any ski area. I highly recommend trying an alpine slide somewhere. There are so much fun. Every summer, we spent at least one day at Winter Park Resort (their alpine slide is the longest). We buy the all-day pass and get there early (just like WDW!) to avoid lines. We'll ride that thing at least 10 times in a day, plus do the other activites. So much fun!
Hiking is everywhere. Estes Park is beautiful, too. You can see the Stanley Hotel (inspired Steven King to write the Shining, and is rumored to be haunted), there's a gondola ride there that takes you to to the top of a mountain full of chipmunks so tame, they will come up to you and take a peanut right out of your hand (and they sell peanuts at the top for you.)
