Sycamore Canyon Park is a wonderful hidden gem flying under the radar in Diamond Bar. We stumbled upon it ourselves while looking for interesting parks to explore. When you first pull into the playground, it's a really anti-climactic view, but if you park at the upper parking lot near the bathrooms you're treated with the opportunity to traverse the layers deep into the playgrounds. There are zig-zagging walkways (universally accessible) that weave their way from the uppoer playground, down into the lower playground. The pathway then becomes part of the playground itself until it finally ends into the walking trails.
Plenty of good shade, and lots of fun opportunities to burn off youthful energy. We'll be back many times because this is definitely one of our favorite places to play.
More at http://www.playsomewhere.com