I'd probably start by writing a will to ensure that my different possessions are given to the right people, and follow it up by killing bad people until I died.
No names worthy of being murdered come to mind at the moment, though; it would take a terrible crime for me to believe I have the authority to end someone's life. So I guess I'd probably decide to do something else instead.
I'd probably spend the rest of my day helping people. When you're about to die, it doesn't matter what you do for the rest of your day; if you want to enjoy it, you will treasure it no matter what you end up doing. You might as well do something that will leave the world behind you a better place instead of taking as much away from it as you can in your last day.