Sitting on the highest peak of Pacific Heights, Alta Plaza Park (literally "high place") provides some views to the north (the bay) and amazing ones to south... and there is not a playground in SF with a better view.
Otherwise, it provides several tennis courts and a basketball court and is very popular for dog owners as well (especially big ones - dogs, that is). Visits on clear nights also reward :)
A couple blocks walk from the north/elevated side of the Fillmore district, Alta Plaza is centrally located and accessible by bus or foot.
Website: http://www.parkscan.org/parks?parkId=17