Anyone familiar with Denver and surrounding areas?

[Shag]

Going to be in Denver at the end of July, and looking for some suggestions on things to do. Currently have a Red Rocks show and a game a Rockies game scheduled, but have a few other days yet to schedule. Looking for a variety of things - outdoors fun (rafting, 4 wheeling, horseback riding), and at least a day of relaxing at some sort of resort/hotel.

- What are some good day trips out of Denver? Don't necessarily need to be back to Denver for lodging - could stay outside of Denver for a night or two.

- Recommendations for some reasonably priced resort-type places in the area? Would love to get out of town a bit for this, have nice mountain views, and a nice pool/springs area to lay out and relax. No kids involved, so somewhere more adult oriented would be ideal.

Side note: the person I'm going with does seem to have some issues with altitude sickness, so if you have suggestions that don't involve going high into the mountains, please include those, as well.

Thanks!