These are my two go to's at the moment for game design.
I use google draw, set up the page to 11 X 8.5, I have a "template" of 8 cards per sheet, and doodle some drag and drop shapes and free clip-art. Plus can make maps/hexes/counters/etc. Might not always be pretty but at least gives me clean, legible, functional, prototypes.
I use Evernote to set up separate to-do lists for Design/Graphic Design/Prototype printing; keep the version history of my rules; and keep a design doc/diary.
Both of these are synced to all my devices and I can work on anything anywhere and it automatically updates/saves. Plus it allows me to check off some things on my to-do lists during lulls at work, since everything can be accessed on a browser.
Are there better platforms I could be using? I don't feel I could do much better than Evernote for the text based side of things; but more than sometimes Google Draw feels.... wanting.