Considering it's SB, land of snooty but devoted foodies, it really depends on your taste. I'm a pescatarian w/ vegan tendancies but a serious love of rich (not so vegan)food. My two favorites...
*Spiritland Bistro: my boyfriend and I went here on our anniversary and just fell in love w/ the place. If you've got a taste for specialty diets (raw, vegan, veggie, gluten-free), this place is #1 on the list. Very small, laid back but elegant, prices are mid to mid-high, andd superb service (they gave us two comped glasses of champagne to assist our celebration, plus two glasses of amazing port, also comped, because they no longer carried a tea I wanted to try). Their wild honey lavender creme brulee is just astounding, make sure you order at the beginning of the meal as they tend to run out. They have everything from the above mentioned specialty diets, to poultry, to EXCELENT seafood, and they do have some limited meat options. Please know this is not your typical "health food"- order off the raw menu and you'll be very pleasently suprised at the richness. Servings run on the small side, but if you do an app or two, entree's and split a desert or two, it's absolutely perfect- light enough to go home and have fun w/ out the food coma ;) HIGHLY reccomended
*Ca'Dario- This is the place for rich, elegant Italian food. Also mid to mid-high (can run higher if you get into the amazing wine selection). Also another very small place, typical cozy Italian feel, and fabulous, fun service (a good deal of the staff is Italian, which should tell you something). Only been there once, but tried the ravioli in brown butter sage sauce- a little oily for my own taste, but the flavor was...out of this world, one of the best pasta dishes I've had. I wouldn't reccomend the bow-tie pasta w/ smoked salmon, I found it rather fishy. Simple, elegant deserts. This is a plae that many of my co-workers tend to go for romantic occassions, and it's a great place for just that. Also a lot of fun for lunch w/ a few friends. Can't remember his name, but theres a tall Asian manager there that is so beyond hospitable- I was really impressed.
The following places I've never been to, but have heard rave (and repeatd) review..
*Olio E Limone- I had this boss who had extraoridnarily high expectations of Italian food, and this place blew her mind, she still raves about it. Prices run mid-high to pretty damn high
*Bouchon- French w/ Cali fusion flair. The above mentioned bosses foodie partner in crime absolutely adored this place. Prices run the same. From his review, their meant selection and prep is heavenly. The two still argue over which is better (my op is you can't compare the two different cuisines, why bother).
For other suggestions, check out the SB Dining guide- you'll have to sift through the stupid all-caps reviews and the really pretentious people who expect far beyond what is remotely reasonable, but The Guide gives a general price line, menu outline and local reviews
Good luck, have fun on your reomantic evening :)