A simple maze design could work - the base could be pegboard, and you could make "walls" out of plywood with dowel rods sticking out the bottom that could be placed in the pegboard holes to make the maze. You could use squarebots alone or herd object balls. Have a dry-erase board listing best times for prizes, or give away chachkis if they beat a benchmark time (30 secs or whatever).
something like this:
At different events you could rearrange the walls, etc. Perhaps make a block P?