While calisthenic training can hit the ceiling fairly quickly for experienced athletes, there are still ways to modify movements like the push-up so that they keep returning value. If a standard bodyweight push-up is too easy, adding a small weight plate on your lower back can bump the difficulty.
Alternatively, implementing “clapping” push-ups requires significantly more explosive power to perform successfully. Finally, a single-arm or even handstand push-up takes the exercise to the extreme end of difficulty without requiring external load.