One cool Boston night tEps set up a kiddie pool filled with nice warm water. A small spoonful of UV reactive dye and it was transformed into a radioactive pit of joy. I will not suggest you do this yourselves. But it is easy to do, get some nontoxic UV dye that they use for flow monitoring in rivers and add it to a pool of your choosing. Bring out your UV bulbs and have fun. Remember to keep it out of your eyes and mouth.
This was a bad idea done by professional bad idea handlers. Please enjoy our mistakes and never do what we did.
As an aside, this type of dye can be used to make invisible spy ink either for silkscreening / pen type applications. Simply find the right binder (clear waterbased/acrylic gel) and write. To see it put it under any UV light source, a bill checking flashlight for instance.