If your mind is blank concerning ideas for a painting then I would suggest a walk around outside for a while. Go somewhere with trees which are thought to relax you. When you get home just go about your usual things that you do. try not to think too much about your painting. You may have an idea or two later on about what to paint.
If nothing else search for some type of paintings online. Do not paint anything exactly like another existing painting. It is just to give you some ideas of what to paint. Best of luck to you with your big canvas.
Robert, I would take it off the easel and place it on the ground over a larger tarp and get some really vibrant colors and do a drip painting or I would do an aerosol grafia outer space painting with some planets, stars and a nebula or two, if curious I have some examples in my gallery.
Some great suggestions, everyone! Thanks to each of you for contributing. Current status: the canvas is still waiting for me to work on it, but it will have to wait for a little while, as I am taking a mini vacation at the beach.