You CAN book post-cruise
DCL Transfers from port to airport during the cruise. It will be charged to your onboard account and I believe they put a sticker on your KTTW card.
I thought you had to sign-up for Onboard Airline Check-in (the service that takes your bags for you) during check-in at the terminal before boarding. I'm not sure that aspect can be done onboard during the cruise.
Is there a reason you don't want to book transfers in advance? Are you expecting your post-cruise plans to change?
Enjoy your vacation!