Chalk it Up
Pick a panel of judges (the oldest members of the family are the obvious picks) and have them become the official judges for a chalk contest.
Break your guests into two teams (or more, depending on how many people are at the party) and give them each one or two containers of sidewalk chalk. Tell them to create a sidewalk picture that shows something patriotic, and tell them the flag must be included in the picture.
Give them a time limit (depending on your group, this time limit might range from 10 minutes to 45 minutes) and then have the judges declare a winner after they have carefully examined all the artwork.
You could have art-themed prizes for the winners.