Now pretend that the kindly old doctor has shuffled into the room, blinking as he shuffles a few papers. "I'm very sorry," he says, "But you've only got about 70 years to live..."

I don't think these are comparable. In particular, the person with six months to live has something the person with seventy lacks: A comparative advantage over the rest of the population with respect to risk. That is, risky actions cost them less.

So my answer to "six months" is: Do things that were "off your list" only because they involved greater personal risk than you were willing to accept. That applies whether you prefer a life of hedonism, utilitarianism, or a mix of both. Go skydiving and also assassinate Hitler (for arbitrary values of Hitler). But your preference for hedonism-vs-helping-the-world shouldn't necessarily change.