A.W.
Althought the throwing paint project would be fun for kids - It's just too much mess IMO for an at the park party.
For the upcycle project, I would cover something that might otherwise get tossed out - thinking cardboard cereal box (use the plain inside part), cardboard or plastic type egg carton.........shoe box? or other small box or pieces of carboard. OH, you could use cardboard and encourge them to make a sign of their name to hang in their room or on their door!!! :)