Shh, don't tell everyone, but hidden in a remote corner of the Mission is one of the best public fields for a private get-together. Call 15 or 20 of your best friends for an impromptu game of football (either the American or Euro kind), or play some fast-pitch on the well-groomed baseball diamond. Tucked away behind the main building of the St. Mary's Recreation Center (which houses an indoor basketball gymnasium that's open on the weekends), the secluded park is a short walk up the hill from the Glen Park BART station, snuggled between the south side of Bernal Heights and I-280. It might be a little hard to find, but that's why the park is so special; trees obscure the grassy field and -- aside from a few dog walkers or the rare softball match -- it's surprisingly private. If you don't want to drag along all your pals, the park makes an ideal hideaway for kite-flying or summertime stargazing. But you didn't hear it from us.