I was reminded of an assignment I had to do in school once. Draw an indoor scene using only a red, a blue (we had magenta, cyan), a yellow, a black and a white pencil crayon. You have to create all the colours by layering.
These are some great ideas! Although, the 100 themes challenge never seemed to help me, I always got stuck or took the theme too literal because my mind would go BLANK.
Another suggestion is to create an "Inspiration Folder" on your computer. Fill it with images that inspire you. Could be other artworks from other people, people you know, or even yourself. Could be photography that just caught your eye and something about it inspires you. Could be just random photos of trinkets or something that you found on etsy that inspires you in some way. Clothing, book titles, poems, quotes, etc. Fill it up! Then when you have a down day, just flip through it and something will stand out.
Don't worry about how big the folder gets, the bigger the better... you can never have too much in your inspiration folder!