If you pay (It was $15 last summer) to Valet park at the Contemporary, you can leave your car there all day, and use the resort monorail to get to Poly and MK.
There may be a similar Valet option at the Poly, don't know for sure, I've never done it there.
We left our car at the Contemporary all day last year, had breakfast at Chef Mickey's, then spent the entire day at MK, no problem.