Along with amusement and fun, you can set a studying plan, maybe General reading is better than just one particular subject, an at the end you will all have accumulated a great wealth of knowledge. Studying together is a kind of fun itself.
Your local library should have a reading program. Our library had a four week program with weekly goals. Each week, you can earn small prizes. For every book you read, your name goes in a drawing. At the end, they draw for big prizes. Last year, one was an iPad. They even have programs for adults.
Go strawberry, blueberry and blackberry picking, and then maybe go on a picnic somewhere you don't usually go. I agree with m360, school being out is a perfect time to try and do some learning on your own, or catch up on books you didn't have time for, maybe read out loud with the family after dinner. Try a lot of crafts, make homemade jewelry, purses out of old jeans, make a picnic blanket out of handkerchiefs, make stepping stones out of big leaves and cement. Go to the zoo, the beach, waterparks, learn to skate, learn to play an instrument, learn to cook new things, have sleep overs or go camping, go fishing, learn magic tricks, buy a ton of board games.
muesuems swimming kid concerts going 2 the parks and have a picnic and go on a walk in the park. maybe have a scavenger hunt of the things you find in the park bowling ice/roller skating laser tag have a themed sleepover for the kids