crayons
sidewalk chalk
puzzle
sunglasses (the cheap ones--check out Oriental Trading Company)
jumpropes (see Oriental Trading Company)

Contact your local library see if they have bookmarks or anything like that they will give you to put in the bags.
If you have any museums see if they will donate any trinkets to put in the bag (maybe even "free child's pass with a paying adult" coupons)
See if McDonalds/Burger King/Wendy's, etc. will pony up any coupons for a free small fries or cone.
Contact any local dermatologists and see if they will provide you samples of sunscreen.
Contact the local hospitals and see if they are willing to donate anything--same goes for Police Dept, Fire Dept and Ambulance service. Most of these have some sort of promotional items that they are usually more than willing to donate for a good cause.

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Mental that one, I'm telling you. ---Ron Weasley, "Harry Potter and the Chamber of Secrets"