Night Time is the Right Time
This one is simple: take whatever you do during the day and do it at night?
Besides being extra fun, night activities carry the extra bonus of drastically lessening the crowd factor. So, don't tell anybody how much fun you are having!
Walk/hike/run - whatever you call it when you move using your legs, do that at night with a flashlight/headlamp. Be safe!
Bike - riding at night typically requires a little extra preparation in the light department. I like a very bright light on the handlebars, plus a headlamp for looking around. We'll try to get some gear tips and suggestions up soon. Until then, just Google "bike at night" if you aren't sure what you need.
Paddle - always be safe! Paddling at night is magical, but don't paddle beyond your abilities. Besides being able to see, make sure you have a way to tell where you are in the dark.
Scuba - night dives are special treasures. Proper equipment and training are worth their weight in gold, so contact your local dive shop if you don't already have what you need.