Best Answer - Chosen by Voters
Make the simplest lowest mass thing you can.
Maybe a rectangular piece of cardboard (or flat plexiglas) about 3" wide by 6 or 8 inches long. It'd have a ½ inch dowel for an axle mounted crossways about an inch from one end and a very narrow (maybe 1/16") slot 4 or 5 inches long in the front for the front wheel on a very short piece of the ½ inch dowel.
I picked ½" dowel as the hole in a CD is just a bit more than this and it'll turn freely on it. A CD hole is closer to 9/16, but that size dowel is probably very hard to find, but if you can it'd work better.
The dowel should be only about ¼" wider than the cardboard. After you attach the dowel to the cardboard, Place a CD on the dowel and then file notches on the dowel so a paperclip can be snapped in on each side to keep the wheels in place.
The front wheel won't need the paperclip as it'll be held in place by the slot in the cardboard.
Blow up the balloon, and attach it to the cardboard with some double stick tape and let it rip. It you make the dowels square to the cardboard and the wheels all are parallel it should go nicely. It'd have practically no drag to slow it down.