https://mitadmissions.org/blogs/entry/applying_sideways/
Do the thing that is truest to you.
ECs related to your prospective major can be part of a strong application- but so can ECs that are completely unrelated. From an AO pov you are showing that you can commit to something over time and learn and grow in it. It can be a proxy for showing self discipline and the ability to organize yourself, and (if relevant) that you can work well with others. Whatever you choose will add color and dimension to your app, b/c it shows something about you as a person.
But to pick your ECs just to impress AOs would be a terrible waste. Do things that are real for you.