Our youngest loves jumping into 'muddy puddles' and looks forward to them every time it rains. Sometimes we'll have her put on her rain boots and take a short walk around the neighborhood to find puddles to jump into. It gets your kids out of the house and doing something a little different!
Another fun activity is to make colorful paper boats and placing them into puddles. This was such a fun thing to do. Toddlers don't have the best attention span and the boats don't last too long so it's actually perfect!
I found this tutorial that shows you how to make a paper boat.