i usually ride redwood until just past the chabot space and science center, then cut across joaquin miller road into JM park, where there IS legal singletrack. cinderella, chapparal, palos colorados, big trees (not really singletrack, but still heeps of fun) are all pretty fun trails. if you know your way around, you can hit redwood, head into JM, hit cinderella, palos colorados, and sinawik in both directions in under two hours. if you want better singletrack in the east bay, you'll have to get a set of lights, go out at night, and be prepared to pay a hefty fine if you get caught.
i recently found out that dimond canyon regional park is also open to bikes, which rocks, because that's all singletrack down to fruitvale, basicallly
a friend of mine just recently poached the whole ridge on a very early sunday morning. from tilden to JM on all singletrack. not a soul to be seen except one jogger, who didn't seem to mind.
it's either that, or cross the bridge to china camp, tamarancho or mt. tam. briones is pretty nice, but not a lot of tree cover, which bites in the summer...and it's alllll fire roads. there are some nice views, and a few technical sections, one trail has like 10 creek crossings, but it's mostly kinda 'eh.' it's probably fun on a 'cross bike though.