Ordinary exercises like pushups and situps can become a bit annoying when you can do more than fifty in a row. Adding more doesn't feel like a accomplishement any more. Somehow it doesn't feel like a healthy activity either.
What do you do to keep your exercises challenging?
I have found that extending the range of motion can drastically increase difficulty. For example take three chairs, one for each hand and one for the feet. Lower your torso below the palms and push up. Repeat. Doing twelve of those is challenging for me. I got that one from Charles Atlas. (Handle with care and some common sense, please.)