A skateboard  which folded into a 1/2" thickness & small enough form factor to fit in a backpack.  The key lions realized was to make the wheels out of 1/4" slices.  The easiest solution is fully disassembling it into parts no thicker than 1/4".  Making it fully collapsible without disassembly wouldn't be possible.

Another idea was just making wheels 1/4" in diameter.  Instead of a board which floated off the ground like the movies, what about a board which stuck right against the ground.  It would look like a snowboard, but skiing on pavement.  Instead of 4 wheels, it would have many wheels.  They would individually brake & be on pivots to allow steering.