Welcome back, Rachel :)
The Cool Down Party is a free event held at Downtown Disney that offers you the opportunity to mingle with your fellow runners as a DJ spins the latest hits and sets the mood for fun. While this is a great way to commemorate your race experience, I'm not sure I would stay an additional day just to attend this function.
I would consider putting the money you would need for an additional nights accommodations toward a celebration dinner for Saturday night at one of Walt Disney World® Resorts Signature Dining restaurants. My favorites include
California Grill and the
Chef's Tasting Wine Dinner at the Flying Fish. A post-race massage reservation a
Senses Spa at Disney's Grand Floridian Resort would also be an option to consider in lieu of any additional night.
Of course, any additional nights in Walt Disney World® Resort you can get, would be worth it, but to stay just for the Cool Down Party might not be the best option.
As always, please let me know if you should have any additional questions. I am here to help you navigate your way through a magical runDisney-cation!
Have a magical race!
Faith