One fun 4th of July activity that just screams "4th of July!" is a parade with decorated bikes, scooters and the like.
Ask everyone who comes to the party to decorate their bike or scooter or other item in patriotic garb. You can have people bring them to the party already decorated and have a contest for "best bike", etc.,
It is also fun is to have a decorating party within the party. Have all the items on hand to decorate the bikes and scooters and skateboards. You might have streamers, banners, flags, and ribbons. The children and adults can decorate their bikes and scooters as a party activity.