Red Rocks / Settler's Park is a beautiful city park on the western edge of town, right at the mouth of Boulder Canyon. It features these awesome red sandstone spires reminiscent of Garden of the Gods (that's the Colorado Springs Garden of the Gods... not Illinois).
Kinetics is an annual Boulder event (this is the 25th year) that features teams racing the oddest contraptions around and across Boulder Resevoir. The Parade takes place a week or two before, and the teams get to show off their crazy crafts and costumes.
The Bolder Boulder is the 2nd largest 10k running race in the world, with 50,000 participants. After the citizen's race, there's a huge (largest in the United States) Memorial Day celebration, followed by the elite women and men's races.