We did a sports day one and it was brilliant. We went to a local park that had a giant adventure playground and held it there. Half the party was doing sports day races (sack races, egg and spoon, etc) and the rest they were free to play on the adventure playground. And eat cake.
MakeitStop, how old were the kids? I have got an open field in mind where I could hold it. I was thinking sack races, egg and spoon, running races, maybe those foam javelin things? What other games did you play?