Although the Pearl Street Mall is technically a mall and therefore somewhat touristy, there are some cool things to check out there. Besides the national chains and local shops, there are a lot of characters roaming around.
While you'll definitely see lots of street and performance artists, a few of them are what makes Boulder so special. For example, check out the work of one of these artists. His creations are only meant to stay up for a day and he gathers leaves, flowers, trash, whatever he can find and puts them on a water wall to make some pretty cool animals and nature scenes.