I don't mind the idea of a notepad, though real writing would better simulate test day as opposed to screen writing.

You might think that a calculator would be beneficial, but if you are truly preparing for the SAT you need to also become familiar with and quick to use whatever version of calculator you will be using on that day.  You can always pull up your operating systems calculator, but to really prepare you should have whatever you will be using and practice with it.  Otherwise, when you solve on the test day the locations of functions and the order of button presses or entry will throw you off your game; not what you want to have happen on a high stakes test. 

I like the idea of a notepad. In Khan Academy's normal math practice/lessons, one can write directly on the question using a pen tool. I like to use this feature for solving problems, sometimes I just need to jot down a number or two to keep track of problem steps.