My main ECs were Chemistry Olympiad and Science Olympiad (swapped out for Science Bowl in senior year). I also did art, but that was a fairly small commitment. In freshman year I was very gungho NJROTC but I didn't get any leadership positions and generally didn't vibe with some of the people so I quit after sophomore year. I tried doing robotics among other things, but nothing else really interested me very much at all.
You can definitely balance Science Olympiad with other things, especially if your school culture doesn't have a lot of clubs which forbid dual participation in SciOly (eg: at my school, you generally can't do robotics and SciOly). A lot of SciOly people did FBLA and went to Nats for that. A good chunk of us went hardcore in other Olympiads as well. Yet others did newspaper and yearbook, though they were basically told to quit SciOly if they wanted to be chief for either of those ECs.
It gets even easier if your events have overlap with other things. For instance, I did Chem Lab, and I never studied directly for it because everything was covered in ChemOly prep. I can also see overlap with BiOly and bio events, or DP Oceanography and Ocean Bowl.
Div D! I really like chem, oceanography, and nail polish--not in that order.
Troy HS, co2016.
Feel free to PM me about SciOly or college or whatever! I really enjoy making online friends.