i grade papers, write lesson plans, spend time w/ the girlfriend & parrot, catch up w/ work around the house...stuff that tends to fall by the wayside when we're getting a run of swell.
i'm lucky enough to have a gf that is VERY understanding when it comes to surfing. if it has been flat for a while, she doesn't mind if i blow something off to go surf. hell, after 2 years, she pretty much expects it.
flat spells are also a good time to catch up on board & wetsuit maintenance...ding repair, strip & re-wax boards, check the suits for tears...normal wear & tear stuff.
in the summer, i'll sit on the beach & read or do something else-swim, paddle, kayak...anything in the ocean, really.