Which would you rather have: a beautiful flower that kills off the entire environment or a skateboard that can hover? (Is this one of those trick questions?)
Hoverboards aren't just the coolest toy in the book, they're also a handy symbol. The hoverboards often get compared to toys or pets: the hoverboard is "like a littlie's balloon" (3.89) or bumps ankles like a dog (4.13, 46.16). So hoverboards = toys. So hoverboards are symbols of innocent childhood, right?
Hold on there. Because in the middle of the book, when Tally is learning how to use the hoverboard, it suddenly seems unlike a toy to Tally: "She had stopped thinking of it as a toy, like a littlie's balloon. The hoverboard had become something more solid, something that obeyed its own rules, and that could be dangerous, too" (9.48). So the hoverboard becomes a tool, something that can be useful or dangerous depending on how it's used. Which makes hoverboards symbols of maturity, right?
Keep holding. Because our favorite villain, Dr. Cable says that hoverboards are always the toys/tools of rule-breakers. When Tally tells her that Shay used a hoverboard, Cable responds, "Of course a hoverboard. What is it about those things and miscreants?" (16.33). So according to Cable, hoverboards = criminal activity. So that must mean hoverboards are symbols of rule breaking.
(Brain Snack: in an interview, Westerfeld notes that, when he was growing up, skateboards became cool. And as soon as kids were using skateboards, suddenly there were all these "No Skateboarding" signs. He points how older people usually don't like the things that younger people do—skateboards, hoodies, the Internet, cat videos, you know.)
So which is it? It's all three—a symbol of childhood, adulthood, and criminalhood. (That last one isn't a word, but we like it.) Hoverboards might be our favorite bit of technology because it doesn't seem all good or all bad or all adult or all childish. So, maybe hoverboards what they really symbolize is potential.