Is it possible to mix a luminescent drink? Is it possible to create a drink that glows? In other words, is it possible to create a non-poisonous drink that fluoresces? Are there known compounds suitable for that? If not, then what about a milder condition: UV-induced luminescence?
DISCLAIMER: I am neither going to drink this by myself nor do any experiments, this is just curiosity. I may only use such substances if they are available from legal producers and are approved by the FDA or whatever.
 A: Tonic water fluoresces - if you shine UV (black light) on it, it will glow.
From the above link:

As a follow-up I did the experiment in my own kitchen - with sparkling water and tonic in two glasses side by side.

The same glasses under UV light:

To remove any doubt - the glass on the right had the tonic water. I removed the white paper background in the UV picture as the paper itself showed some effect of the whitener added...
A: Try fluorescent dye (non-toxic, please!). Highlighter ink can work for ordinary tap water (more detail here). Fluorescent paints and glowsticks can also produce decent amounts of light; however, all three of the above methods do not produce substances that are safe to drink. Always be careful when using dyes and other chemicals in beverages.
Putting rum on the surface and igniting it also seems to work; this, too, is dangerous, even if you don't drink the resulting mixture!
For anyone who's curious, this is what the flaming rum looks like (probably not what you were going for, admittedly):

A picture of the water glowing from highlighter ink (mage used under Fair Use):

