Ideas Needed

Will will be done with this year of preschool next week. So with the rest of summer looming and four children to educate and entertain seven days a week, I need some input. I asked him what he wanted to learn about this summer, and he said, "Mama, I need to learn more science and more math and you should teach me how to cook." Ah, a boy after my own heart (and his daddy's). He is getting better with numbers and simple math. Last night I told him when he learns to add and subtract money, he can get an allowance. He was so excited,
"When I get money, I can buy my own house!!"
"We probably won't give you that much of an allowance."
"Okay. Then I'll go to the refinery and work"
"When you are older, you can do that if you want to."
"Yeah! Then I'll see you, Dad, while we work."

So, to encourage his love of learning, give me some ideas of how to incorporate science while entertaining the babies (19 months now) at the same time. You see, when he does anything, they want to participate and that often gets Will frustrated. I need some activities that can keep them busy and Will entertained & learning at the same time. We don't get a lot of time when the babies are not around, just the few moments they are napping.