Great question!!! Especially, as you say, on a day like this!
Given that traditionally most opposites are not the most different things you can think of (black and white are normally said to be opposite, but they are both colours) - but instead the most extreme ends of a spectrum, I would say that the opposite of a tomato is a potato ... the plants look very similar - but one gives you something red, sweet and refreshing to eat raw above ground ... and the other gives you something white, starchy and earthy to eat underneath.