I know we're favoring water and somewhere nearby, but I would think that he would opt for burial and somewhere further. After all, the "abduction" did not occur in Fremont, but a half hour away in a different city. If he wants anyone to believe an abduction took place, I would think it would benefit him more for a body to turn up closer to Oakland than to Fremont. (What are the odds an abductor from Oakland heads to the exact area the kid's from to dispose of a body?)
Also, the Scott Peterson case got such huge national play, and that was an attempt at a water disposal at Berkeley Marina, bodies found not too far from there in Point Richmond bay. Also, the Fremont area is pretty marshy with low water, which may have made it harder for water disposal. I'm guessing he had a lot longer than one day to figure it out.
Other ideas, for whatever they're worth:
the Sierras
Sacramento / Delta area
Marin County open spaces / Sonoma County (lots of land)
I'd be VERY interested to find out which bridges he crossed between 7/31 or therabouts (when text message was sent and neighbors confirm seeing Hasanni) and 8/10. It may tell us about any unusal trips into these areas being that you have the Richmond Bridge, Golden Gate, Dunbarton, Carquinez, Benicia, Antioch, Bay bridges to name a few.