I'd probably try to write down as much as I could. I don't so much care about not existing after death but the thought that keeps me up about five nights a year

I'd probably try to write down as much as I could. I don't so much care about not existing after death but the thought that keeps me up about five nights a year is that death is a damned waste. You spend your whole life developing a mind and accumulating knowledge. You experience things, learn how to navigate familiar and unfamiliar situations, and hopefully you achieve some measure of success as you grow. All of this has immense value to other people because it's already been paid for with the dearest currency of all - time. And then, poof - one day, it's gone. The pattern of your mind is gone, and nobody else can learn all those lessons you've had to learn the hard way. That's what keeps me up at night, and that's why, if I knew I was going to die tomorrow, I'd spend every waking moment I had telling strangers things I've learned so that, hopefully, they wouldn't have to learn those lessons themselves.