A buddy of mine and myself challenge each other with just a plain sheet of paper. The idea is to cut the paper in half horizontally. Once we do that, we cut it in half vertically.
I like that idea..
I sometimes use a large paper or cardboard square, like 18" or more, with no markings, not pasters, no bull's etc. I like to use something like that at a pretty good distance, 25 yds or more. Forces me to aim at the PERCEIVED center of a big square instead of aiming at a little dot. See how many rapid fire shots I can get into a small group. Rapid fire so that I don't tend to take time to aim at the previous bullet holes.
On the first day of the week at my range (Wednesday) there's a lot of target holders with brand new, non shot up cardboard. When they have those I shoot at the imagined center of that 3' x 3' square.