If it's summer then I generally go body surfing as there is usually just enough swell to do that. I don't do it in winter as going body surfing in a wetsuit seems a bit weird, and you can't lie in the sand afterwards for an hour like in summer. If it's completely flat then sometimes it's a sign to get things done around the house that you keep putting off.

There's also (heaven forbid it) the SUP for the small days, not my cup of tea.
Most things will cost money but if you invest wisely you can have a lot of fun on one purchase.
Here in Sydney flat days in the summer/spring often have quite a lot of wind so it's an option to get into sailing, kite surfing or wind surfing. I've got a 14' Hobie Cat for days like that. There's also the snorkelling, or, spear-fishing option. Then there's normal fishing, or fishing in a cheap tinny.