Giant granny square blankie .. basically just keep crocheting until it's big enough or you're sick of it.
There are some great tutorials on the Youtubes. Meema Makes is good - https://www.youtube.com/watch?v=9X0sLr6Wpfs
I usually have one on the go because stopping to crochet to 10-15mins a few times a day helps me not get RSI from too much keyboard time.