Is this for this year? You're running out of time!
But for the future, our elementary school does a Field Day with a series of activities. Some are for all to enjoy at once, and others are set up as "stations" with the classes rotating through them.
Each class works with the phys ed teachers to prepare a routine. They use props and perform a dance-type routine (not a lot of dance skill required - more of a moving formation sort of thing). They put a popular and upbeat song on, and perform in lines/circles. Props for one group might be hula hoops, for another group might be small colorful scarves, and for a third perhaps ribbons on a stick (similar to rhythmic gymnastics) - not expensive. The prep becomes the phys ed curriculum, working on movement and coordination as well as cooperation with others. Three kindergarten classes, for example, would learn the same routine with the same prop, then perform on the field in 3 adjacent circles or line formations, with the other grades watching. So it doesn't take forever. Each group wears the same color tee shirt (purchased, or supplied individually - families borrow and trade so no one has to buy if it's not affordable).
Stations include classic field games like tug-of-war, potato races, sack races, 3-legged races, and so on. There are a million party/field games you can use. Our phys ed teachers are fantastic at preparing these, and there are great camp/party books you can find in the library with neat things that were done all the time in the 50s and 60s that these kids have never seen. There are also stations appropriate for all kids, including kids who aren't able to do the races well: beanbag toss into a grid, rubber ducky floating games, guessing games, and more.
Then there are popsicles and ice cream treats, lemonade, and so on. Again, this is done in shifts.
