Park City is an amazing destination for mountain bikers of all skill levels, and Round Valley is one of their flagship outdoor attractions. The trail system is huge, located just on the edge of town, and it allows a wide array of possible uses.
Bring your dogs and let them run the entire area off-leash, ride your horses along the wide tracks, or bring the most commonly seen tool: bikes. Fat bikes. Hybrid bikes. Mountain bikes. They all pass along the rolling scrub oak and small trees. Seen everywhere in the lowlands of these basins, the open scrubland gives you great views of the neighborhoods and ski resorts of Park City.
The area is popular because of its proximity to town and wide range of difficulty levels, but it's large enough to handle even the weekend crowds. People from Salt Lake City make the journey up in the summer for cooler temperatures and high-quality trails. There are 700 acres available to explore in any season, which gives you a combined 30 to 40 miles to play with. An easier ride would be to start out on Fast Pitch Trail to Round Valley Express, which gives you a steady loop of 6 miles with only 200 feet of elevation gain.
Singletrack and doubletrack trails mostly stay in the green but have some blues mixed in. The far north and west sides are more rocky and technical, but there's nothing too intense. The paths closer to the trailhead are very well groomed but also more crowded. The exposed terrain gives you an early start and late finish to the season, and even on the hottest days of summer it doesn't get too bad at this elevation. That said, there is no shade.
Quinn's Junction is the main trailhead, but there also is Round Valley Way, the Cove, Old Ranch Road, and Highland Drive. The sports complex at the trailhead includes a beautiful soccer field, fenced-in dog run, bathrooms, a ton of parking, and an ice skating rink. Everything here is of a really high quality. Keep an eye out for the National Ability Center near the trailhead, a great non-profit that helps people overcome various challenges from physical to financial.