I wanted to do a large landscape, so I used the largest canvas for that. For the other painting, I did a floral painting. You could also do something more abstract. The important thing is, if you mess up or don't like it, you can always paint over it again. I actually did on my original floral painting. I didn't like how it was looking so I started again from scratch. No big deal.
I used acrylic paint for both of mine. I added oil pastel on top of the floral. I sealed it with a spray varnish.
The great part about upcycling an old painting is that it is so much cheaper than buying and framing a canvas. I do this a lot with framed art prints. Definitely give this a try--you'll be pleased!