You can incorporate this activity with the sidewalk chalk activity since they both require outside time. Outside activities are usually more fun and attract the attention of children more easily. For this activity you will need small water balloons. Fill up about 50 water balloons or so ensuring you have enough depending on your class size. Divide you class into two groups making sure the groups are even in number. Have the two groups arrange themselves in two straight lines directly across from each other as they will need to have a partner. Have the partners toss the balloons to their partner. It will take the children a few tries to get the general idea of throwing and catching the balloon properly so that is not dropped with every attempt.
Rather than the traditional game of water balloon tosses, this easier version will not disqualify a partner couple once the balloon is dropped but rather reward each set of partners for each successful toss. Small pieces of candy, stickers or other incentives can be used. This activity would best take place on a hotter day when children wouldn't mind getting wet, though it is not intended to be a water balloon fight.
Another activity if you are using the sidewalk chalk is drawing different shapes such as rectangles and circles at different distances then having the children toss the balloons into them, aiming for the shapes as targets. You can use the two groups used for the water balloon toss and the group that has the most successful throws that make it into the target wins.