My first thought, obviously, was masturbate. But i suppose i should put in more effort than that...
I'd say kill someone i really hate, but that would probably take more than 15 minutes if i'm at home...
Humdeehum... This is hard, but that's because i'm trying to be serious...
Ah, got it, i'd spend 5 minutes on a quick will, then i'd step outside and kiss the first attractive woman i see (with no warning). Tell her i've got about 8 minutes to live and see if she fancies a quicky. If she says yes, we have amazing sex and i die happy. If she says no, i go back inside, masturbate and i die happy.
If there is an afterlife i will meet said attractive woman there (regardless of whether she said yes or no, she died of sadness at the sudden loss of the best thing to come into her life... duh) we would then proceed to have amazing sex for eternity...
Or until i get bored with her and play Minecraft for eternity, stopping only to have slightly less amazing sex.
Clearly i cannot lose... Y'know, except for that whole dying thing.