Pearl Street is a great area to explore near both of the hotels we've suggested! There are so many shops, restaurants and coffee shops to visit while you're visiting Boulder. (One of our favorites is Liberty Puzzles).
Chataqua is a beautiful park at the foot of the Flatirons in Boulder. There is no fee to hike in the park and it's a great way to get active and see some beautiful views!
Visit the iconic music venue for a hike, a beautiful sunrise/sunset, or - if the timing works out - for an amazing concert!
This link has a huge list of hiking trail options in the area with a wide range of hike difficulties, as well!
For anyone interested in white water rafting, this is one of the closest options to Boulder. They have different intensity options ranging from beginner to intermediate-advance! This company also offers activities like zip lines and ropes courses!