If you want to give your credit card some exercise, head out to Pearl Street Mall in Central Boulder.
If you want to see a show while you're in town, try and grab some tickets at Boulder Theater in Central Boulder. Why not take a stroll along the riverfront and watch the sunset while you're here? It's also full of great shops and restaurants, so you can spend the afternoon having a wander. If you enjoy the show and want to see another, Fox Theater, Dairy Center for the Arts, and Upstart Crow Theatre Company are not far.