$20 for one person? Not for a family, right?

My pick would be a scooter for a day, if you can get one for $20. Actually that would work for 2 people as well. Ride around, stop by Jurassic Park or Brighton or ride all the way to Little Bay. You'll need about $2-3 in gas though.

Second choice, glass bottom boat out to reef to snorkel or along the cliffs to sightsee.