You need the ap/voucher to check in, although this is debated as some have never been asked to show anything, but better to have it, imho, just to be safe. Yes, you can buy them at the gate, but not at the resort, or at least that is what I've been told...so someone correct me if I'm wrong!
I guess if you had a car it would be ok to stop at a park to do this before checking in, but a hassle w/ no car.