I love to have my phone out on PokeGo while I'm shopping in supermarkets! :-) The concentration of cellular and wi-fi activity seems to make markets decent spawn areas (in total numbers; the diversity seems to be the same as the area around).

I do sometimes worry that I'm liable to do something foolish in the midst of this, though, like absentmindedly set down my phone in my cart while I pick up a grocery item, and end up with my phone vanishing...