Check out Netti's crabshack. It's bright, airy, walkable from your area, and has some of the best seafood in the City. They have a whole crab dinner (choose roasted or steamed, sides, etc) for around $30. Great wines and cocktails too.
Nettie's Crab Shack
2032 Union Street, San Francisco, CA 94123
Isa and Bistro Aix, both on Steiner between Lombard and Chestnut are both excellent. Terzo on Steiner near Union is also a favorite of ours. For Contemporary Vietnamese, Three Seasons is on the same block as Isa and Bistro Aix. Mezes, near A16 on Chestnut is a small but very good Greek restaurant. Near Betelnut, which someone else recommends, is Netties Crab Shack and a very new French bistro, Des Amis--haven't eaten there yet but have reservations in a couple of weeks--it is owned and backed by people with other great restaurants so I expect it will be good. They put a ton of money into it.
2355 Chestnut St., San Francisco, CA 94123
2030 Union Street, San Francisco, CA 94123
2373 Chestnut Street, San Francisco, CA 94123
3317 Steiner Street, San Francisco, CA 94123
3340 Steiner St, San Francisco, CA 94123
3011 Steiner St., San Francisco, CA 94123