Never stayed at YC, but you'll love the Poly! I know it's weird, but I love the SMELL of the resort! I came home and unpacked, and I had a travel pack of tissues. When I picked them up, I got a whiff of that Poly smell from them, and got transported back to WDW!!!
Eat at Kona's for lunch or dinner, especially if you like food with an Asian flare. For breakfast, many people rave about the Tonga Toast there (I found it too sweat, but it WAS good), and their french press Kona coffee (if you love coffee you'll DIE over this!).
Go down the water slide. It's fun! The pool itself is a lot of fun, and has zero entry (like going into the ocean) so it's fun to watch the littles splashing at the waters edge.
Take the boat at least once to MK, from Poly it's a non-stop trip. You can walk from Poly to the TTC to pick up the monorail to Epcot, this will almost always save you time if you don't mind the walk (and it isn't that far from most longhouses).
Walk around the grounds at night, the tiki torches make it so romantic.
You may want to skip the Luau, most people don't care for it. I hear the food is bad to so-so. Never tried it myself though.