Don't know how soon you are planning this (because of the heat right now), but what about a picnic or cook out at a park? You could have Pizza delivered or you could cook hamburgers and hot dogs on the grill. The older kids can play at the playground or you could have croquet or volleyball or other games played on the grass while the littler ones can play on blankets or their age appropriate outdoor games. You could do relay races for the older kids, too.
I think this will also give your friends from California the chance to visit and get to know the other adults in the area. Have fun!