I'd be playing as much as possible to soak in all the joy I've experienced in talking to others and looking at other stuff... also roleplaying. It'd be depressing to see the game shut down, hopefully not soon though.
And perhaps if this is after I released my map pack, I'd explore around the maps myself for the last time. Even if I know every place. I'd probably do a lot of things to make the last memories of the game itself.
I don't know since it's not ending any time soon exactly, but I would surely miss it if it was down forever.