Since I don't believe in it, I'll go at it hypothetically, as if the whole world knew for certain it would happen.
First thing I'd do is get a hold of a nice car to drive around during preparation. Say all the chaos unfolds a week prior to doomsday. I'd get in touch with my friends, my family. Arrange something for us to stay in contact up until the end. The last hours would be reserved for my family. Heck, I'd be surprised if anyone else wouldn't at the very least have them there with you. I'd probably get my closest friends and their families all together somewhere nice.
Anyway, second step: Heroin or opium. Why? It's obvious. Heroin will provide me with the best, stimulation/strain-wise, feeling I'll ever obtain. There's a reason people do it, you know. I'd get a hold of a supply that would last me, and anyone who'd care to join me, all the way up until the end. Given the end of the world I wouldn't have to worry about the come-down, which is the reason it destroys people who do it, and furthermore why I'd never ever try it.
All the people I love and adore, the most potent happiness-inducing drugs... just come together and talk about our past lives and all the great times we've had. All together in some nice, warm climate, or somewhere nostalgic. I'd also eat plenty of good food, have a lot of ***, stretch out, give/receive great massages. Bottomline: Just pursue all the greatest feelings in the world, obtain as much pleasure as possible. Cause at this point I wouldn't have to worry about any of it being long-term.