Science olympiad. Ever hear of it? Its a national school science competition in which you specialize and compete for medals and trophies. Out of high school? No problem! Just volunteer for one of the events. You can be a referee of sorts.
The competition is open to middle and high school students. You generally progress from regionals to states, and if your team is full of prodigies, to nationals. You choose from one to six events from about 25. You study science concepts that are usually exceptionally advanced, and go to a tournament to test your knowledge.
Why this can count as a hobby is that the range of topics is very large. You can study biology, physics, and geology. You can try your hand as an engineer and build a tower out of balsa wood. There are so many options, and chances are there is some remote corner of science you will enjoy