Chow - Church and Market, solid modern-American food (not cheap, not fancy), nice patio, on the noisy/busy side
Millenium - probably hard to stay under $25, but amazing romantic atmosphere and super gourmet veggie food - maybe for a future date =)
Ti Couz - 16th and Valencia, mouthwatering delicious crepes, pricey but probably under $25, a little noisy/busy depending on time of day, and I think no reservations unfortunately
Nirvana - on Castro by 18th, heated patio, modern Asian/American foods, pricey but good sized portions and probably under $25
Bissap Baobab - yummy Senegalese food in the Mission (near 19th), some dishes can be veggie, in your price range, good music, and has a rockin dance spot around the corner if you stay around late enough
Golden Era - great completely vegetarian Vietnamese/Asian food, near Union Square, not sure about atmosphere (I think I've always gotten take-out), reasonable prices
Benihana?? I haven't been there in years - maybe it's tacky, but they chop/cook food in front of you, and one out of SF i went to recently was delicious and really reasonably priced
I would also look for Indian restaurants, buffets, as they're often good for veggie