I'm a 3rd grade teacher, and I use Khan Academy ALL the time in class. 

However, one difficult concept for 3rd graders, rounding, does not have great skill breakdowns in its available exercises. I would LOVE some exercises along these lines:

--Fill in the missing number on the number line (KA has this skill for 7th graders with negative numbers, but not for a positive integer number line). It would be awesome if this exercise could include questions like "Which number is 63 closer to?" With 60 and 70 as the choices.

--Round to the nearest ten (currently the only rounding exercises have mixed questions for both tens and hundreds). It would be especially awesome if this could have breakdowns from 0-99, 0-900, 0-9,000, sort of like the addition exercises. It would also be amazing if there could be different exercises for numbers that end in 5 and numbers that don't end in 5.

--Round to the nearest hundred (as before, right now this is only available mixed up with rounding to the nearest ten). 

That's it for now! If there is something else I can do to help with this, I'd be all about it.