Note: you'll see this effect any time you have a tonic-containing drink under a blacklight. See How to Make Edible Glowing UV Reactive JELLO, for example.
Step 1: Ingredients
I bought the flashlights from DX as "UV Money Detector LED Flashlight Keychain 10-Pack" for $5.24. Allow a couple of weeks for them to show up. You can buy them at the dollar store, too.
Step 2: Build & Serve
Give one to each guest along with the UV LED. Once everyone's set up, switch off the lights. They'll figure out what to do next :)
Enjoy! Video shows what you can expect. You'll get 99% of the effects in the first 30 seconds, so no need to watch the whole thing.
I skipped the lime and ice for this demo. The ice causes condensation on the glass makes the picture a little fuzzy, at least on screen. However, don't skip it when you try it - you get additional effects shining the light down through the ice.
If you liked this drink, you might enjoy The Color-Changing Martini.
Other drinks involving tonic water and blacklights can be found in the ebook Shocktails.