There is a $2/person entrance fee for the park. This is a multi-use trail, be alert for other trail users and be prepared to yield.
The Liberty Lake Loop Trail is more frequently traveled by hikers, but it's also a fun option for bikers. Just know that you'll have plenty of company during certain times of the day and especially on weekends.
The trail begins as a smooth stroller path and advances to lung-and-leg crushing singletrack and switchbacks. There are some fun punchy sections before it gets serious. Though at some point you'll probably end up hike-a-biking. The descent is steep, wide, straight, nontechnical, and just engaging enough to keep you interested. I rode it clockwise, but could see myself doing all sorts of variations depending on my mood. Each section of trail is very different.
If conditions are warm and you have the time, the beach near the trailhead makes a nice post-ride cool-down.