St. Petersburg is a sea port and there are a lot of places to feel the soul of the sea and enjoy the sun and the beach, even if it is not a summer now. My favourite place is Beach Laskovyi close to Solnechnoye. It is easy to get there even with public transport: every half an hour there is a train from Finland Railway Station (check the Finnish Railway station article) and at almost the same frequency a small bus (marshrutka) from Chernaya rechka, Prospekt Prosveshenia or Staraya Derevnya underground station.
The beach is sandy and full of pines and firs and some stones and I enjoy going for a really long walk along the Finnish Gulf. There are some restaurants and cafes perfect for a break and to enjoy some lunch close to the sea shore. Alternatively, you can organize a barbeque party just on the beach.
Enjoy the solitude and peace during the winter season and the happy crowds swimming, sun-bathing and playing beach volleyball during the sunny weekends of the summer.