Hiking and trekking are almost a religion in Boulder. Boulder Open Space and Mountain Parks oversees more than 43,000 acres of open land and 148 miles of trails. Two of the more popular trails are Bobolink and Marshal Meadow.
Another website, Boulder Colorado Hiking also has descriptions of many of the hikes in and around the city.
Both sites have, or lead to maps of the trails.
The hikes from the NCAR trailhead are popular, The Colorado Chataqua National Historic Landmark, is a wonderful park with several great hikes, including some that are good for families with younger children. (End your hike with brunch at Chataqua’s Dining Hall.)