KHWD has only one FBO.
AvGas is very expensive (check Airnav.com) but for that price I think they will let you park for free, and probably give you a ride to/from BART. You may want to call the FBO (see Airnav.com) and confirm the self serve price. Last time I looked you had to have some sort of an account with them to get it?
Check out KLVK. Gas is at least $1/gallon cheaper, but they do charge ($6/night, I think) for transient parking. Depending on what time you are arriving/departing I may be able to give you a lift to BART. Otherwise, there are taxis and a city bus to BART.
Hayward's FBO says free parking with fuel purchase, no minimum. So maybe the best deal is to stop enroute but only an hour or less away, then at Hayward tell them to top it off knowing it won't take more than 10 gallons.
By the way, due to traffic, KLVK is closer to a BART station (in time) than KHWD is.