Rain shouldn't stop you having fun. It was C.S.Lewis who wrote that people who didn't like going out in the rain probably had mothers who wouldn't let them play in it when they were kids.
But what to do on a sunny day? I recently saw a cartoon in (I think) New Scientist which showed a couple sitting on their porch watching the sun explode. The man said, "Wouldn't you know it? The first day in weeks we get fine weather and the sun goes nova."