Our field day turned out real well this year, we had tug of war, sack race, wet sponge relay, water balloon toss. The kids really enjoyed it, especially the wet sponge and the tug of war. In the gym we had a basketball shoot out and a baseball throw (throw the ball into a cut out on a board).
Last spring I literally got the whole "funday" dropped in my lap with about 4 days notice. We did the following events - kickball, volleyball, relay races, tug of war, face painting and "childhood games" like duck, duck goose and red rover. We also did a scavenger hunt. The kids had a great time and we didn't have to invest hardly any money because we took the sporting supplies from school. The kids loved it because it wasn't complicated.
Our kids have loved it each time we have a karaoke machine, and also during some of our events we have had laser tag and video game tournaments. For prizes we use simple stickers. Believe it or not our lower el kids preferred a sticker to a ribbon, especially if they got to pick the sticker they wanted.
Our Field Day has been split so that half the kids are taken to a nearby park in the morning for the regular Field Day activities, then they'll return and the other half will go to the park during the afternoon. The PTO has been asked to think of some activities for kids when they are back at the school. So far I've come up with face painting, dancing, crafts, and board games. Any other suggestions (should require minimal preparation and inexpensive supplies)