nigirisushi (a piece of raw fish on rice) is pretty simple and healthy -- if you like variety, go for a chirashi (a bowl of sushi rice with a variety of raw fish on top of it)
California and Spicy Tuna Rolls are generally pretty healthy -- but beware of any rolls with fancy names ("Dragon Roll" "Las Vegas Roll" "Hunky Hunk Elvis Roll" ... ok, maybe I made up that last one

)-- those generally are ladened with avocado, teriyaki sauce, mayo and/or deep-fried tempura.
If the sushi place has a picture menu -- it's that much easier to avoid the fatty ones.
And if you do have the opportunity (and budget) to try quality ootoro sushi (fatty tuna), I'd highly recommend it. The calories & the $$$ are well worth it in my opinion.