The Magic School Bus kits are a lot of fun. My kids are 4 and 6 so I have to do it with them, but an 8 year old should be able to do them more on his own. Check out the Young Scientists web site. They have a Magic School Bus Science Club. We get a science kit in the mail every month. We also get a monthly newsletter and access to a members only section of the web site. It is about $20 a month, but if you watch Groupon you can sometimes get it for 1/2 price. I think you can also get 3 month or 6 month memberships. We have kits for magnets, volcanoes, light, states of matter, fossils, air, the human body, fungi and bacteria, and weather. You can also buy individual kits on line and in teacher supply stores.
We also have the That's Gross Science Lab by Smart Lab. It would be a dream for an 8 year old boy! It has a toilet bowl mixer that is the basis for a lot of the experiments like the Foaming Flush. You can also make Gooey Pooey, edible dog vomit, Snot Slime, Cabbage Crud, Gaggots, Yellow Snow, Popping Poo, Stink Bubbles and more! It is a riot!! My 6 year old daughter and her best friend love it.
The best thing about both the Magic School Bus kits and the That's Gross Science Kit is that you can do the experiments over and over (for the most part). Almost all of the experiments use pretty common household items. The items in the kits are usually things like funnels, measuring cups, balloons, paper cut to the right size. The items are usually reusable things or ones that you can easily replace without having to special order them. Sometimes it is a certain amount of something like Borax so that you don't have to buy a huge box for the 2 tsp you need for the experiment.
We also have raised butterflies and lady bugs. The Insect Lore website has butterflies, ladybugs, ants, and I think now they also have preying mantis and worms. You can buy most of the kits at places like Toys R Us, local owned toy stores or teacher supply stores. The kits come with a coupon to mail in (or order online) to get the caterpillars or larvae. My kids have loved doing both the butterflies and lady bugs. They are easy to do and don't take a lot of work to raise. And you can release them once they have grown. The habitats can be used over and over and it isn't too expensive to order new critters the next year.