Wyndham Canterbury is clean, has nice décor, and all the convenience you want -- free wifi, small, but functional work-out room, and kitchenette complete with microwave, fridge and even wine glasses! I would definitely stay here again for my next conference.
The hotel has a great location, with walking distance from most forms of public transport. Union square isn't far from the hotel either (I walked there to get to the Apple store and it only took about 10 minutes). There are plenty of coffee shops around as well as other general stores. The staff are always friendly and ready to help. On check in, they will help you with the area, giving details of the local stores, area and a map locating where they are in relation to the rest of the city.
Pleasant staff. Large clean rooms. Family of five fit comfortably. Close to freeway and grocery store. Was a great 'home base' for our forays into the city. Included breakfast and occasional meals in the kitchenette tampered the high price of eating out in San Francisco.