Load up the water pistols and have a water fight. If you want to create some structure, divide the family into 2 teams, set a time limit and lay down rules about, say, where players can go (e.g. not inside the house and not outside the property boundary), or how many times they can refill their weapons. Alternatively, just have a free-for-all skirmish and see who’s left standing!
Great fun, especially on a hot day!