[EricH]

Build a robot (doesn't have to be for a game, but a working drivetrain is handy to have lying around for defense practice).
Offseason events.
Train new members.
Do an engineering challenge--try a punkin chunkin event or some similar idea.
Strategy concepts--use old games and develop a strategy to win.

Of course, this is just a short list of options--some teams don't do any, some do all or most.