I think the secret to sweet tomatoes is plenty of sunlight, and the secret to sweet
Morelle De Balbis is prolonged cold weather; the secret to sweet tomatillos seems to be to let them ripen all the way (i.e. fall off the plant; one of the sweetest ones I ever had ripened in the shade). The secret to sweet muskmelons seems to be the same as the secret to sweet tomatoes.