I'm generally not a fan of UV lights or playing with flouro things in them. (My eyeballs seem to like laser/LED/big lights kinda stuff. Plus, UV makes my auto-tinting eyeglasses go fog-white. It's annoying. Walk into almost any club, and go blind.)
But playing with superbright flouroscent marking string in UV has to be one of the coolest things I've seen in a while. (Hey look, I'm playing with string! wee!) Get a good length between your fingers, twirl it like you're going to jump rope, and then set up mutliple oscillations. You know, like how you do with string? Ya give it a nudge, yeah? Harmonics. Get'cherself like three or four waves going in there. Lime/yellow does it for me. Haven't tried a braid of multiple colors or patterns yet.
I've also seen it wrapped/spiraled directly around UV tubes for great glow effects.
Come to think of it, I have seen cheesy kinetic blacklight string sculptures at those places where they sell that kind of useless crap, along with the whoopy cusions and tit-shaped beer helmets that say "Nipples!" on one side and "Beer!" on the other.
But it's just not the same as sittin' there playing with string.