Sonofusion [0] was a neat idea, and some of the relevant experiments (at least the cavitation part) can be done on a bench top with a hobbyist level budget. Last I checked, the research had been somewhat tarnished by academic fraud. The physics of jet formation during bubble collapse near a solid boundary remains fascinating [1].

[0] https://en.wikipedia.org/wiki/Bubble_fusion

[1] https://authors.library.caltech.edu/48933/1/On%20the%20Mecha...