I live in North Beach (right by the wharf). I'd recommend going on Yelp and picking out some restaurants that suit your fancy...tough to go wrong in SF. The Stinking Rose is a famous restaurant in North Beach that uses insane amounts of garlic but is incredibly delicious. As far as Marin, I'd go out to Stinson Beach if the winds out there aren't too bad...some nice hikes/walks out there too. Bahiker.com is a great resource if you're looking for hikes to do. You can rent all kinds of bikes, "go-cars", segways, etc. down at the wharf to explore the city (and beyond) on as well. Great mexican food in the Mission, Ocean Beach/Sutro/Legion of Honor area is beautiful....basically the park around the SF side of the golden gate bridge. May is potentially a great time of year to visit....as you probably know the summers here get pretty cold with our "built-in air-conditioning". For some delicious and casual Chinatown food I'd recommend either Henry's Hunan or the famous House of Nan King.
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