A fairly short list for me I think:

MIA - to and from South America
SIN - on the way from Thailand to Japan
GYE - changing planes to and from Galapagos, from Quito (I guess you could get there via a BA codeshare on AA)
ALF, TOS, HFT - stops on the little northern Norway hopper which is more like a local bus service (not BA destinations)

For everywhere else I have always tried to take advantage of long layovers to at least go and spend a few hours in the city, but I am mainly travelling for leisure so I can take my time.