Have ya thought about getting addressable RGB "pixels"? Since you can address each one individually, an idea is that you could have a strip connected to each bike that connects to the top of a tower. And as people cycle, you see a conga line of colors ("electrons") "travel" up the individual strips to the tower faster or slower depending on how fast they pedal. Then having one placed vertically in the tower that gets "more full" as "more electrons" are dumped into it from the individual bikes until it finally fills above the red LED signifying enough power has been generated for that one device.
If you have just a few LEDs lit on the strand at a time, it would be in the 5v, ~200ma range per strand.
Another idea is to have a grid of LEDs above everyone and the more people pedal, the more white LEDs flicker above them, randomly spaced apart; signifying the more stars we see as we depend on fossil fuels less.
All that being said, these are certainly a much more impressive idea during the nighttime. If you're stuck to day, I like the cardboard pointer idea