Call up the family and friends and take them to a theme park and then out to eat at a restaurant. Then tell everyone goodbye.
Dunno if I'd tell them they'd never see me again or not though. Depends on if they're running off crazy too much or annoying me
Wouldn't wanna spoil the day.
Would try to casually bring up the topic of "if I die I want..." in regards to items that needed taken care of - my pets mainly, but also reminding that I want my body donated.
Speaking of, might consider going to a hospital right before death, depending on how/what I'm dying of, so they can harvest organs.
I suppose if the theme park idea is too much (due to money and time - I have a very large family and we don't actually live near a theme park so it'd be quite a drive) substitute in something else, though hopefully more special/memorable than just spending all day at the park or river since that's basically what we do for birthdays and general get-togethers lol
Maybe skydiving - I'm terrified of heights, so that's not something I'd do normally or likely enjoy that much, but it would be memorable