2 liter water bottle rockets are really fun on a summer day. There are a lot of different launch pad designs, but kids will love that. Here is one for $55. This one can be a bit dangerous. Remember you are launching a 2 liter bottle of water 50+ feet into the air.
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Lego ramp racers. Have some legos on hand and give the kids 20 minutes to build and test a lego car and then race them on a ramp. Very similar to Pine-wood derby, but with the build aspect it allows for a FIRST build season twist. Take a picture of the car and write the trap time on the picture and hang this on the wall of your booth.
Egg drop. Have a random supply of materials and and allow the kids to make egg drop designs. Weigh the total system and do a drop height divided by system weight score. Take pictures of the system before begining the drop and then post the photos and scores on a wall in your booth. Top score at the end of the fair wins a prize.
Paper-airplane flight for distance and accuracy. Give each kid a sheet of paper to make a paper airplane. Draw a line on the ground and have the kid throw their plane. Divide the length thrown by the distance away from the line. Write the score on the plan and hang it on the wall of your booth.
These ideas give a very organic feel to your booth. Kids (and adults) enjoy doing these things and then bringing their parents by to show them their picture, plane... This give you a great chance to talk to the parents about FLL, FTC, and FRC.
These are some idea that are good for almost any age. What age group are you targeting? What kind of space/facility do you have available?
The egg drop is good because it has a small footprint with a very exciting visual. It also requires the least set-up as really all you need are a bunch of eggs. Some random materials, and a ladder. You may want to construct some sort of drop tub to help with the splatter of poorly protected eggs. A clear sterilite tub works with a metal plate in the bottom works really well. It is a bit wasteful though so don't tell the green police I told you to do this.