Pearl Street Mall
Pedestrian Street mostly with lots of shops, art studios and places to eat. Will often have some street performers as well. From 8th to 20th street.
Chautauqua Park
Park area located at the foot of the Flatiron Mountains. If you want to hike this is a good place to go - you can just meander around the initial walking trails or brave a steep hike up to one of the Flatirons. Then if you wanted you could drive up Flagstaff mountain for a pretty drive and nice views.
Boulder Creek
A beautiful creek that runs through Boulder. Many access points. Enjoy the sun and the water - lots of areas to set up a blanket for a picnic. You can also get water sports rentals at various locations in Boulder. The creek is likely to be cold and fast-moving, however, due to the mountain snowmelt.
29th Street Mall
Another outdoor mall you can walk around. Some great shops (Anthropologie, Sephora, etc) and yummy places to eat!
Nederland
A lovely mountain town with cute shops and great hikes! Only 25-30 min from Boulder and the drive is through a beautiful canyon. On the way up you could stop by Boulder Falls, a cute waterfall in the canyon.
Estes Park
Gorgeous town! One of the entry points to the National Park, it has shops, a creek, a lake (can rent paddle boats or pontoon boats), and places to eat. Lots of elk and sometimes moose – STAY FAR AWAY, they are NOT friendly. The scenic route to Estes is on Peak-to-Peak highway and is gorgeousl, but takes longer than the direct route from Boulder.
Rocky Mountain National Park
Requires reservations in advance, and has an entry fee. If you head into the National Park for hiking be advised there might be snow on the ground in areas - microspikes are advised (you can rent them at stores in Estes Park). If you cannot get a reservation, you can hike Gem Lake which is just outside of the National Park.
Denver
Denver is about a 30 minute drive from Boulder and has some good food, drinks, craft beer, and fun things to see (like Meow Wolf, a fun, immersive art exhibit). Google is your best friend for recommendations!