Just to add another thought, I agree with grimley1968 that they might close down some transportation modes before you want to leave, but the worst-case scenario is that you'd have to take a cab back to the resort you left your car at, and because all those MK resorts are so tightly packed, the cab fare between any two of them is rarely more than $5, including tip.
So driving to the various resorts is certainly an option, and not a bad one. But you could valet park at the Contemporary, take the monorail or boat to another resort, walk around, have a Dole whip, take in the sights. When you're done, if the monorail is running, take it back to the contemporary. If it's not running but the boats are, take them to the MK and walk to the contemporary. If everything is shut down, just walk to the front of whatever resort you are and get a cab back to the contemporary. They'll drop you right at the porte-cochere, where you can hand your valet ticket to the guy and they'll bring your car.
If it were me, I'd do the second option, because the process of getting into your car, driving a short distance, checking in at the gate, getting another parking pass, finding another parking space, etc. is less magical than just taking a monorail or walking. And once you get to the Poly or the Grand, you can walk to the other one, which is a nice walk.
But really, neither option is bad. Follow your gut! Have fun!
