When I get an orange juice on the plane, I'll always ask for lemon slices/nubs too. Sometimes if I remember beforehand, I'll grab a cup of lemons at an airport bar, which I'd use for water too.

As for foods, pretty much any candy that's meant to be tart/sour, particularly "Sour Patch Kids" and the sour watermelon pieces, lemon drops (though those are often more sweet than necessary), Jolly Ranchers watermelon and lemon, and tamarind candies are nice too. Key lime pie for sure, and definitely grapefruit.

Yoghurt too, but that depends on what I'm eating with it.