Marlie202, your sweet/plain lassi is right on. Salt lassi that my mom makes includes chilled yogurt, ice, salt, small pinch sugar and a large pinch of ground toasted cumin(toast a teaspoon or two of cumin in a hot skillet stirring often until aromatic and crisp. Let cool for 5 minutes and then crush with a rolling pin into a coarse powder.)
There is a thinner yogurt/buttermilk drink native to western India that is just as refreshing as lassi. It is called "mattha" - I paraphrased a recipe here:
It is similar to AnnaEA's version above, except for the addition of ginger and sugar or honey.
I make a mango lassi sometimes - plain yogurt mixed 50/50 with a can of thick mango nectar - the nectar is almost but not quite a puree. Thinned with a little water if needed.
I also make (for myself, and my Uncle Lakhi - no one else likes it!) a salt lassi -- yogurt thinned with skim milk or water, very lightly salted, and with a little bit of cilantro or fresh green chili stirred in.