You can just take it easy at MK, as others have mentioned take advantage of the shows and airconditioned areas. You could make an ADR for a late lunch to take a long break for lunch either at the MK or maybe a monorail resort.
Alternately if you haven't checked your luggage through with ME via a participating airline you will still have access to it. You can still head back to the resort for a break and a swim, and a rest by the pool. They resorts have showers and lockers for changing. If you have checked your luggage through just pack a daybag with what you will need and bell services will hold it for you until you need it.
We have also gotten a late check out, sometimes as late as 1pm. You may
have to pay a fee for it but when our dds were very small it was worth it.
TJ