I was in town for business and only had about an h...

I was in town for business and only had about an hour to explore the park, but I must say, in that short time I was impressed. The park itself is free to enter and there is a 1.4 mile loop you can hike through the red rocks. It is beautiful.
I did not make it to the amphitheater but would hope to return someday to see it. However, there is a pretty cool museum / gift shop that shows you a lot from past shows. There are pictures and memorabilia you can look through while you are there.
The gift shop prices are not too bad. About $20 for a shirt (some a little more, some a little less).
But hey, if you have limited time or all day, this is a great place to check out if you are in the Denver area.