It would be difficult to put out too much scare during the parade since I expect there will be many young children. Bloody, graphic, or shocking depictions would anger parents.
Maybe something more subtle and ominous, like a portend of things to come. But also something that can showcase your abilities. Here's an idea:
The center of the float has a long "hallway" that hides the leaping creatures (I forget the technical name, but the one that rises and extends on armatures) hidden behind curtains. The entire outside of the hall is your ad banner. Manually controlled at various times, a section of the banner parts, and suddenly a leaper comes out, extending towrds the crowd. Could use a giant spider, skeleton, alien, etc. Use smoke machines, lights, and sound effects to amplify. A actor as the Grim Reaper could be on top of the whole thing on some sort of crypt looking alter, pantamiming (sp?) to the crowd to a pre-recorded monologue about how he, "...will come on all Hallows Eve to..." and, "...enter is you dare..." and, "...only the brave will survive...", etc.
Maybe have some actors in full costumes walking and acting alongside the float to showcase your costuming. You could time a few routines with the actors and leapers coming out of the float for a fun effect on the crowd.
Maybe add some water-spitting props to cool the hot summertime ToT's-to-be. Be sure to hand out spooky candy and knick-knacks to the crowd! Have a spooky music/sound fx score playing the whole time.
Not sure if I described clearly, I hope it was understandable.
I just re-read your first post and forgot that you had an animatronic skelly set up. Replace Grim Reaper with Skelly. Skelly could be talking (mainly to himself) on how to "...operate this crazy thing...", pulling random levers to see what happens. That's when the leapers could come out.
"What does this button do?" (another skeleton gets flipped in the air with an accompanying "I can see my house from heeeeere!".
Any pyro effects allowed?