Explore Rocky Mountain National Park. One of the many entrances is located in Estes Park approximately 1 hour from Boulder. A beautiful drive or hike will be waiting for you. Breathtaking views all over. Nowadays, a park reservation may be needed if going in the morning. Starting August 1st, September reservations should be available.
You will find reservations and information at www.recreation.gov.
Otherwise, after 2pm you are able to enter at will.

The Chautauqua trail, in Boulder, is a great way to enjoy the outdoors and view the famous Flatirons! Its a 1.3 mile out and back trail. It takes an average of an hour to complete. Parking can be tricky so if needed, transportation is available at https://bouldercolorado.gov/trailhead/chautauqua.

Make sure to bring lots of water whenever you are hiking!

Added bonus: make sure to eat at the Chautauqua Dining Hall!

A 4 block pedestrian mall full of shopping, dining, and many watering holes!
Definitely a must see in Boulder!
Be prepared to run into some street performers!

A must see in Estes Park! Explore the grounds, stay in a haunted room, or do a ghost tour. The Stanley Hotel is best known as the birth place for Stephen Kings, The Shining. On a lighter note, you could grab a drink at the bar where Lloyd Christmas from Dumb and Dumber learned we landed on the moon! :)

Just 20 min south of Boulder you can hike, bike, or climb amongst the sandstone cliffs of the canyon. The park is open from sunrise to sunset all year long! Pack a picnic with lots of water and explore this state park for the day! Due to the park's popularity you may need a reservation. To make a timed entry reservation visit: https://cpw.state.co.us/placestogo/parks/EldoradoCanyon/Pages/vehiclereservations.aspx