We just had a weekend full of kids. There were three kids in the house, in addition to our little one. It is a lot of work, and I don't know how people do it, but man it's fun!
We played board games. I tought them some programming. We played chess. My wife took them swimming. We even borrowed this huge projection screen and a projector from our friends, made popcorn and had a real movie night. Admittedly, the movie was mindnumbingly silly, but hey! That's kids! Silly, silly, silly. And it's great!
Borrowing a lot of kids for a few days is fun. Do the grandparents thing, and return the group when you're exhausted. I highly recommend it.
Now all I have to do is keep a safe distance from my wife until her rekindled baby fever wears off...