Too bad my girls are already in bed, as I know they would have some great ideas.
Watermelon is in season. It's inexpensive and goes a long way.
You could also do a snack mix by putting a bunch of everything in a large bowl and mixing (chex cereal, golden graham cereal, animal crakers, pretzels, m&ms, dried fruit, goldfish crackers). Use what's on sale.
Update
I asked my girls for their favorites.
The walking tacos as described in another post, use nacho flavored doritos.
Banana Boats - leaving it in the skin, slice a banana in half lengthwise, stuff with marshmellows and chocolate chips, wrap in aluminum foil and heat in the fire
Wicky Wacky Walkaways - fill a ice cream cone with pudding