A great and unusual weekend activity, and a very memorable date idea, is hang gliding. The closest airfield that provides this is Highland Aerosports on the other side of the Bay Bridge.
Hang gliding is incredibly safe and very fun. Its quite thrilling to be high up in the sky, with a completely unobstructed view, and to be able to pilot the glider slowly to the ground.
There are two ways to do hang gliding: either strap in a glider and jump off a cliff or have an ultralight plane tow you up and let you glide down. Luckily for us there are no mountains close by suitable, so at Highland Aerosports they use ultralights. This is much safer as well as being far less nerve racking.
Flights start at $145 for gliding down from half a mile up. This takes about 15 minutes and is absolutely worth it. Remember to bring sunscreen as there isn't any permanent shade in the landing field.