After the Flatirons, Mount Sanitas just to the north may be Boulder's second most popular hiking destination. Sanitas offers a few trails which range in difficulty from mild strolling to somewhat more challenging climbs.
The 3 mile loop may be the most classic route here, which, being a loop, conveniently leaves you right back where you started. You'll get some decent views of Boulder along the way, and the walking isn't very strenuous. For the more adventurous, there are also a few cool bouldering spots around.
Lots of walkers bring their dogs here, many of which are let to run off the leash, so don't be too surprised if you see one thundering toward you.