The tasting room with a fire going. The bookstore that doesn't mind if you stay two hours. The museum nobody on TripAdvisor mentions. Rain on a lake is one of the better problems to have.
A show cave on William Penn Highway with two connected cavern systems — winding passageways, flowstone, and thousands of stalactites, open daily year-round. The rainy-day answer for the Huntingdon end of the lake, not on the water itself.