Calcio did a great job of giving you the ins and outs of the transfer. It is easy, and pretty painless.
Just make sure you have everything you need for the day before they take your bags. They send them out a bit later in the day, it isn't an immediate transfer.
If it is a park day, I'd suggest Epcot. You can then jump on the monorail to Epcot, and then you can just boat or walk back to YC from the park through the international gateway (about 5 - 10 minute walk - very pleasant) This way you could always go over to YC anytime during the day to check in early (room might not be ready, but at least you have a room key for charging privledges, etc.) MGM is also fairly close.
If you are going directly to the YC, I'd jump on the monorail and catch a bus back to YC from the MK. You can check in and then enjoy a nice day at Stormalong Bay (our FAVORITE pool!) There are restrooms for changing into swim suits. Just remember that your room may not be ready until after 3pm. Once you are in your room, call bell services, and they should be able to bring your bags right up.
Have a great trip!