Decorations can be red, white and blue bunting with lots of balloons.
Or make an Olympic Rings cake. Hang them from blue, red and white ribbons to place around the 1st, 2nd and 3rd place winners in each event. For the events, if you are not too picky about your lawn you can spray paint the race tracks on it or find chalk used for chalking sports fields. Every participant gets a medal and photograph for this event so no one is left out. Any type of 'event' can be used for this party.