Would "proficient" be useful, if not that, "competent".  Both words suggesting a comfort with tasks given within a certain range.

What is a *slightly* less extreme equivalent to being "fluent" in a language?
Are there any interesting, important or (for the non physicist) astonishing examples where the recoil principle (as special case of conservation of linear momentum) is applied beside rockets and guns?

I am teaching physics in high school and was just wondering if there are any more interesting examples than the two textbook standard examples above. 

I am not interested in examples which are not in the domain of macroscopic, classical physics like Mößbauer-effect or something like that.
