There is a multitude of uses surrounding this park. I used to walk around the lake, roughly 3 miles round (2.8 or 3.2 depending which trail used), to get outside and be surrounded by people doing the same thing. Primarily, I enjoy the people watching.
Aside from the lake, there is a great kids playground in the north side of the park. It is always filled with kids and parents find this a great meeting place for playgroups. The playground is across the street from the Greenlake Public Library and by the Greenlake Community Pool.
I like to go here and catch a drink or meal across the street on the north side, or vice versa. You can bike ride, Rollerblade, walk, jog, run, walk your dog or take the kids for a stroller ride. There are athletic fields, boating, picnic, wading pools available. ( http://www.Seattle.gov/parks/parkspaces/greenlak.htm )