This happened when we had my daughter's 5th birthday. We had umbrellas and the kids thought it was great to run around in the rain with the umbrellas.
We also filled small paper cups with water and partially froze them. Then we put a quarter in the cup and filled it more and then froze it solid. We had a race to see who could get the quarter out first.
Draw on the wet cement with chalk. Use washable markers to draw designs or pictures on paper. Then set the paper in the rain and watch how the picture changes when it gets wet.
Play volleyball with balloons.
Race to see who can pop balloon the fastest. (Just be sure you clean up all the balloon pieces. They are hazardous to birds and small animals!)