What makes Boulder  so appealing is easy access to the great outdoors, where you can simply start running, biking, or hiking right out the front door.
Chautauqua Park  is a great place to start for hikes of varying length and difficulty. Go to the Ranger Cottage near the entrance of the park (Mon.–Fri. 10 a.m.–4 p.m., Sat.–Sun. 8 a.m.–6 p.m.) for park maps, brochures, and expert advice from the staff. Royal Arch, which is less than a mile one-way, is easy and rewarding with a great view from the arch. This is also a starting point to reach the base of the three flatirons, which can be climbed too.
For a bit steeper climb, Gregory Canyon is pretty and thick with trees before opening up for beautiful views. Go to the www.bouldercolorado.gov  website for trail maps, closures, where dogs and bikes are allowed, and other helpful information.
Known for rock climbing and natural springs, Eldorado Canyon State Park (303/494-3943, www.parks.state.co.us/parks/eldoradocanyon ) is also a lovely place for hikes. It is just south of Boulder  about 10 miles, so it’s reachable by bike or car. There is a fee of about $3–7 to enter the park on foot or in a car. Trail maps should be available at the entry station.
Mountain biking is allowed in Eldorado Canyon, and there are 43 miles of trails around town too. Again, the site www.bouldercolorado.gov  provides trail maps and tips for riders. The popular Greenbelt Plateau Trail offers a chance to see a bit of wildlife, birds, and probably some equestrians. It’s an easy ride on the plains below the foothills and connects to other trails.
Cycling on paved roads and trails is nice along Boulder Creek, where the path stretches out east of town to Cherryvale Road from its starting point in the foothills at FourMile Canyon. The trail is very popular and there are speed-limit signs on this seven-mile path.
There are a few options for cooling off in Boulder  on a hot summer day. First, the Boulder Reservoir (5100 51st St., 303/441-3468, www.ci.boulder.co.us ) east of town is Boulder’s version of a beach. It’s a shadeless, sandy park along the water, fun in the summer for sailboating, swimming, fishing, or just floating. Boulder Creek is used for kayaking and tubing, but be aware of the rough conditions before trying either activity. The creek is very choppy in spots with large rocks.
If you just want to hang out with the little ones and enjoy a playground, the best options are Chautauqua Park , where there is a playground and large grassy field; Scott Carpenter Park (30th & Arapahoe Sts., 303/441-3427, www.ci.boulder.co.us ), which has a large outdoor pool, skatepark, and playground; and Eben G. Fine (3rd & Arapahoe Sts., www.ci.boulder.co.us ), with a playground, picnic spots, and a chance to wade into the creek.