The Flatirons

These jaw-dropping rock walls flanking Green Mountain have become a city symbol, with many Boulder businesses sporting their image as a logo. Flatirons One through Five are the most prominent, but there are also several smaller formations -- with more creative names -- to explore. Altogether, these picturesque slopes are the most popular rock climbing areas around -- you can even see markings left by some of the first climbers (dating back to the 1940s) on Number Three.

But for those of you who aren't quite comfortable entrusting your life to a harness, you can still take in the Flatirons from one of Green Mountain's hiking trails (these magnificent rocks line the east slope). However, recent visitors advise that you be prepared for rough terrain. According to one TripAdvisor user: "Trails are short in distance, but they have steep grades, which can be hard on the knees. I recommend going slow or bringing a walking stick if you have joint problems."

Overlooking Boulder, the Flatirons are free to visit every day from sunrise to sunset. For more information, visit the Boulder Open Space and Mountain Parks website.