Balls, jacks, art and craft supplies like popsicle sticks, glue sticks, beads, foam letters/numbers, water squirters, coloring books or coloring pages that you print, old magazines/newspapers with instructions to cut out certain words and glue them to paper, scissors, foam picture frame to put a picture of themselves or their family taken during their summer break, tic tac toe or dots sheets(should be able to find these online to print), rubber bracelets.
Walmart has an area in the toy section with 88 cent toys. Some have multiple toys in a pack, like stretchy frogs or snakes. Dollar General stores usually have a good summer toy section also. And as already mentioned Dollar Tree is usually good for goody bag type toys.