Gates has a pretty active challenge ladder. Summer is busier than fall, but there were a dozen guys playing 5.0 this summer and 40-50 playing 4.5. You can go to http://www.gatestenniscenter.info/
and see the ladder list (password is "summer" or "fall" depending on season). Most guys communicate via email, so I'd go that route.
If you're interested in hitting next week, let me know. I play on the 4.5 ladder here, but will be free Thursday or Friday evening next week. I'll bring the high altitude balls.
Comeback, there are plenty of clubs in the area, but Gates is public so you only pay for what you use. They keep a pretty active schedule of drill classes going too. Court times are $8/hour prime time and $6/hour the remainder. Downside is they don't have indoor courts, but the weather in Denver is such that I've played outdoors in shorts on Christmas days several years. It could also be a blizzard, but tennis outdoors is pretty well guaranteed 10 months a year.