I did an Engineering degree in undergrad similar to yours. I wasn't 100% set on medicine when starting undergrad and I wanted to have a good "backup". Anyways with regards to balancing EC's vs course work. I picked a few key things that I enjoyed doing for EC's and stuck to those. Obviously you should do as many as you can handle, but don't sacrifice your GPA for it. GPA is always going to be King (and its going to stay like this for a while). Concentrate on doing well, and EC's will come with time don't worry.