When I was younger: peeling the foil part of a Club or similar wrapper away from the greasproof backing, in one go, with no rips. Not now, because a) I can't afford Clubs and

the wrappers seem to have changed and the two parts are more tightly bonded, and you can't get them apart.
Now, I think I'd have to say something like cooking something I cook a lot, like spag bol, or spag carbonara, but hitting just the right amount of everything, so that it's absolutely spot on perfect tasting. Mainly, getting just the right amount of pesto on the pasta, or getting a carbonara sauce just the right consistency (IE not too runny, not scrambled eggs).