When we had weeks of rain, I brought indoor sand castles to my little one by the means of Moon Sand (click to make it here). Now the sun has finally come out and all she wants to do is jump in muddy puddles, thanks to Peppa Pig.
After an hour of planting beans in the garden, she took it upon her herself to pour water into her tuff tray and then spent another thirty minutes jumping in it and adding mud and leaves.
So it seems whatever the weather you can go to the beach on a rainy day and jump in muddy puddles on a sunny day.
Moon Sand made from baby oil and flour: