There are 3 courses relatively close to Boulder:
Bird's Nest (64th Ave & Hwy. 93) - A relatively flat 24 hole course. A lot of birdie opportunities...can be windy. Not a lot of trees and/or obstacles but some fun holes and little chance of losing a disc. It's nice to get in 24 holes, too.
Westminster (Church Ranch Blvd. between Wadsworth Blvd. & Sheridan Blvd.) - 18 holes, some open, some elevation changes, not a lot of trees, does have a creek that comes into play on several holes, can be windy.
Badlands (90th & Camenisch) - The best course in the Denver Metro area. 27 holes, elevation changes, long holes, a lake that comes into play, a "back 9" that is VERY wooded. Just about everything you could ask for in a disc golf course.
Also relatively close and worth the drive is Johnny Roberts (58th & Allison). Short and tight course with a creek that comes into play on about 15 of the 18 holes. Wear sandals or bring a disc retriever. Also, might want to get there early if the weather is nice...the course is very popular.