scootnashville.com mini-cards with the holes punched in 'em...
* Using rubber bands is best. Clicking the photo above will probably clue you in to how that works, but ask if it's not clear. Packs of rubber bands are available at most stores (Dollar General, K-Mart, etc.) for $1 or so, and what is good about them is they're easy to place on a stranger's scooter (stretch over the mirror, and leave dangling from the mirror stem) without damaging the scoot in any way AND it's very easy for the "victim" to remove, which IMO is pretty important. The goal here is to attach it to the bike in such a way that if the rider doesn't immediately notice it, it stays with the bike long enough that they eventually DO notice it... but can remove it without tools.
* Small nylon zip ties (like the pink one, above, obtained at Dollar Tree; imagine it through the hole in the mini-card instead of the rubber band) will also work, but require a cutting tool of some sort to remove, which might piss-off some riders, and that's not good. My recommendation: If you're going to use nylon zip ties, before using it, cut into it about halfway with nail clippers or diagonal cutting pliers (think electronics work) so that it can be easily broken with fingers. Also, only tighten the zip tie to the first few notches, to leave room for a finger to get between it and the mirror stem, so it's easy to remove. Tightening a zip tie to snugness is a VERY BAD idea, here, okay? That makes it a bitch to remove, even with tools! That's why rubber bands are more user-friendly.
Let's go taggin', and bring in the n00bs!