One of the greatest warm-weather activities I've done was called Diving for Sunken Treasure. We bought an inflatable swimming pool (6'x3') and set it up in the yard. Once it was filled with water, we dumped in a few bags of Nestle Treasures candy (they look like little treasure chests).

The kids then took turns in the pool, one at a time. They were blindfolded and had to find and pick up a piece of candy using only their mouth -- which requires them to dunk completely under water.

They were given a time limit, and however many chocolates they could find in that time, they could keep. This was fun to play and fun to observe while waiting your turn! Also, these chocolates tended to stay well-wrapped and avoided water damage.