Another (less known) characterization of planar graphs is Schnyder's theorem, which characterizes planar graphs according to order dimension. That is, a graph is planar if and only if its incidence poset has order dimension at most 3.
Also, I would be remiss to not mention the beautiful (strong) Hanani-Tutte theorem:
A graph is planar if and only if it has a drawing in the plane such that every two non-adjacent edges cross an even number of times.

