Subjective enthusiasm welcome!
Add in cheap controlled motion (think $200 CNC engraver machines) and you can build a comparator[3] to do precision measurements that used to require expensive machines.
[1] https://graphics.stanford.edu/projects/array/
[2] https://graphics.stanford.edu/projects/array/videos/crowd0-s...
[3] https://www.instructables.com/Comparatron-an-Affordable-Digi...
GNU radio is open source, and will work with your audio ports... so you can shift audio up/down in frequency, make a sonar, etc... all for free, and in your spare time. Add in a $30 RLTSDR dongle, and you can receive almost anything.