Typically when I eat Indian food I put a spoon or two (or three) of yogurt on the side. Sometimes I do this just for added flavor but a lot of the time I do this to help cut the spice. To add even more flavor to the plate you can spruce up the yogurt.
Tomato OR Cucumber
No real measurements here, just do it to taste. Chop the tomatoes (or cucumber if you are using this) and red onions into small cubes. Mix with a bowl of yogurt and add a pinch of salt for taste. Let it "marinate" for awhile. Spoon onto rice/Indian dish. You might even get experimental and try it with other dishes. I've been known to use yogurt in place of sour cream with my tacos - so there is no reason why this yogurt/onion/tomato mix wouldn't be good with tacos/burritos, I would think. I think I'll have to try that sometime. What else can you think of that this would go well with?
(The best red onion for this is the small kind but I imagine any sized red onion would do. I know there is a name for this small red onion as I read about it in the February issue of Martha Stewart's Everyday Food Magazine, I think it starts with a P?)