A "clinical experience" EC where you have contact with patients is practically required(shadowing, ER volunteering ect).
Research varies by schools, some really want you to have it, others it looks good but isn't so essential.
After those two I think most people just do stuff they like doing(not that you can't like the above two things), or work for money.
Something I got asked about in alot of my interviews was leadership experience. You can get this through clubs/organizations, work and probably other places as well. These can probably be found in something your already doing, so it's not really necessary to do something just for leadership experience.
As far as time goes: They perfer to see long term commitments. So, it would be better to do something a few hours a week over a long period, than to do something for many hours per week over a short period.
And you are correct, don't sacrifice MCAT or GPA in the name of ECs. Although you will need both for a competative application.