I actually find one of the best buoyancy exercises is to hover and try to stay perfectly still -- I mean STILL. No hand movement, no fin movement at all. If you can stay perfectly still for one minute, you're doing very well -- Now try it with your eyes closed!
Other very good (and even better, useful) exercises are to do all your skills while hovering. Flood and clear your mask, and try to stay in the same place and at the same depth. Take your mask off and replace it. Switch from your primary to your backup regular, and back. Do an air-share with your buddy. Try to do all these things without changing position or depth.