When it comes to upper body, my 2 favorites are clearly the L-sit to handstand and the front lever. But even exercises as rad and fun as these can get stale once you've mastered the final steps (full front lever and 10 L sit to handstands).
A simple but awesome way to modify these to make them more challenging is to simply change the hand position. Lately I have been really grooving on these uneven L sit to Handstands:
You can actually do the same thing with front levers starting over from a tuck lever again by holding on to a martial arts belt to change the elevation of the assisting hand.
For a harder variation on the front lever, you can also bring your hands very wide and do anything from front lever holds to ice cream makers.
Just changing the hand position will not only keep it fresh and interesting but it an integral part of keeping your joints healthy and preventing overuse injuries. Explore the space and don't be afraid to color outside the lines.