I like the answers about spending time with people I loved and trying crystal meth. I could see myself going two routes. I'd focus on something I want to change about society and be very aggressive, vocal and definitive about getting it done. Technically, if we know the date we're going to die there's no risk of dying any sooner.

This might give me the freedom to push further and more forcibly for something I believe in.
On the other hand, I've always had this far-out dream of dropping a lot of acid, doing a lot of intense mental and physical conditioning and basically trying to see if mind expansion is real. I like to think that years from now we'll have a lot more control over LSD or nootropic drugs, so that we won't freak out when we take them but will instead reap the benefits of that heightened awareness and hypersensitivity to one's surroundings.
Or I'd rob a bank, Inside Man style.