Not that I have kids of my own, but Ants on a Log immediately comes to mind when I think about after school snacks. I like the way you have to hold them delicately between your fingers, and the way they sanction my peanut butter habit. Other than that, I brought them to an Oscar party this year and they went over swell.

Having sons, I know that they need something hardy. I usually try and bake something that will give them energy like pumpkin or banana muffins made with yogurt and canola oil.  I try to keep sugar at a minimum.

PB & banana, PB on celery sticks (a la Ants on a Log, above), PB on some whole wheat bread. Keeps up the energy and tides them over until dinner.

Is your peanut butter organic?  Just asking because peanuts are one of the most heavily sprayed with pesticides.  It sounds like you eat a lot of it, so just some friendly advice.

I agree though, peanut butter on cardboard would be good. 

Make some tail mix with almonds and dried fruit.  If you don't mind it, throw in some semi-sweet chocolate chips (healthier than the milk chocolate....) and viola!  A tasty after school snack high in protein, healthy lipids, and fruit that the kids will enjoy!  Let them pick out the dried fruits to make sure its a hit.

My kids like granola or luna bars.  Sometimes we cruch them up and sprinke them over yogurt.  Delicious and satisfying.  Crunchy yet creamy too.  Texture makes a difference.