The Blue trail is easily accessible, is great for hiking/biking, and has some great views across the lake.
Here are links to the official park site & to a trail map:
http://www.mercercountyparks.org/parks/mercer-county-park
http://www.jorba.org/sites/default/files/2010_SMART_trail_map_0.jpg
If you haven't noticed yet, the park has done a great job marking out the trails. Whether it be with colored signs or wooden borders along the trails.
ENJOY the natural beauty!
Oh.... and bring a writing utensil.