it depends on if by minimum you mean minimum and still have a good chance of getting in several places, or minimum to have the slightest chance of getting in anywhere at all. to have a decent shot at getting in a few places if you apply early and to plenty of schools, I'd go with a 3.3 and an 18 minimum. however there have been people who have gotten in with like a 2.9 and a 19, but I am pretty sure those people have some kind of special circumstances or connections and I wouldn't count on it.
research, shadowing, volunteering = it depends on your grades and dat, if you have great stats then you don't really need to have much in the way of extra stuff to get in somewhere, but if you have the minimum stats then I think you'd need a good amount of volunteering and other stuff to show you've been doing something productive with your time... but great extracurrics can never do for you what good grades can when applying, so I think your time would be better spent trying to make good grades than tons of extracurriculars.