Rock collecting
Radioactivity
Meteorology (designed a few instruments)
Marine fishkeeping, particularly elasmobranchs but telosts are much easier to keep
Optoelectronics, mostly coherent optical lasers of 530 nM and shorter. Even *cough* x-ray *cough* lasers
Recording audio, restoration of good old analogue recordings
Boating
Collecting estate timepieces
Cheers!