Light up your cooler with some waterproofed LED lights. They're simple to make and easy to reuse.
Step 1: Light it up
All you need is a couple dozen 10mm diffused LEDs and some CR2032 batteries. Clip the leads a little and slide the battery up between them. If it didn't light up, flip it the other way.
Now tape it up and repeat until you have a small pile of glowing lights.