The one in the UK that I like most is EGGW for.... well, Luton, obviously. I mean, who could ever find that misleading or confusing?
Not ALL US airports "just add K". Hawaii and Alaska are PH and PA respectively. Though conveniently, Honolulu (HNL) becomes PHNL and Anchorage (ANC) becomes PANC. There is also a tiny handful that just don't follow the rule, can't remember them offhand. Could be interesting if anyone ever started a commercial service into Henderson, Nevada (which is a GA reliever for Las Vegas) - KHND. I guess something would have to change and it probably wouldn't be Haneda (RJTT, but you guessed that already).