Join a pool! If they can take swimming lessons, they’re sure to meet people to swim with afterward. I know I did that most summers as a kid. I was also lucky enough to have beaches nearby.
One summer my mom pulled out a bunch of graph paper and markers and told us, “We’re each going to invent our own planet. It can have any kind of climate you like, and you can draw a map of it on the graph paper. Then you can invent the culture, the language, the music, the history – whatever you want!” She made one too, along with us.
I was eight, I think, and my brother and friend were eleven. My planet was called “Mariah” and was all ocean with islands everywhere, where everyone spent all day on the beach.

I got to give a little “report” on it, and the older kids HAD to listen – a big bonus for me! I never thought of it as educational at the time, but I guess it was. I just liked the chance to make something up.
I believe that game kept us totally entertained for days. We also had tons of other games we played: building blanket forts, having “sock-ball fights” (perhaps a little violent), cards, board games, Legos, playing spies or house or X-Men in the yard, studying bugs, building a treehouse… Even on long car trips we had games we played to entertain ourselves.
Most of the time we made up our own games … with no TV allowed and no computer, and the knowledge that if we complained about being bored we’d get more chores, we were still never bored. My brother and I were talking about it once, and we both agree that we have hardly ever been bored in our lives, because you can always come up with something to entertain yourself. I believe all kids have the ability to do this, you just have to turn off the TV and let them at it! Even if they are different ages and genders, they can still play together, at least sometimes.