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.
Step 2: Start the Wrap
Tear off a square of plastic wrap and push the LED light up into the middle of it.
Now pull the rest of the plastic wrap back so it looks like a plastic comet.
Step 3: Tie It Up
Twist the tail a little tighter and knot it up. You can trim the excess off or leave it.
If you have some small Ziploc-style bags you can use one or two of those instead. You could also try doubling back the plastic wrap and make it more secure. I just had a bunch of LEDs, batteries, and drinks lying around so I put this together and it worked for the few hours I needed it.
Step 4: Toss Them Into the Ice and Stir
Spread the lights around the ice in the cooler and stir it up a bit so some of them get under the ice to create the glowing effect.