There's KBDU, the Boulder municipal airport - stay outside a two nautical mile circle of them. Otherwise just don't fly it from a park in the actual city, outside of that you should be good to go. There's class d airspace to the south. Just use B4U fly or AirMap and you should be good. I fly in Boulder a bunch. Happy flying!
Just a heads up, to echo what PeterG posted, you can't fly from a city park. Including the park at the base of the flatirons (it's named Chautauqua); many people think that it is a state park. Accoridng to the Rangers if you launch off of the property you can fly onto the property… But you cannot launch or land in the park