Definitely do EMT if you have time to train and work while you're in school. Being an EMT will definitely help you gain experiences with patients and teach you how to be quick on your feet!
1.) Summer camps:
Be a camp counselor for children with disabilities!
I highly suggest this. You'll have so much fun to bond with the kids and be outdoorsy to boot. At the same time, you'll learn how to take responsibility for another person, a mildly vulnerable one at that. They might need help eating, taking meds, getting on a zipline etc. It's a great way to stay active during the summer while developing that much-needed compassion for humanity.
2.) Research programs:
A summer research program may be helpful to see if you're just interested in committing to research or want to think about a MD/PhD. Or if it's a niche research that you just love and can't find a P.I near you, go for it!!! I usually just suggest this if you don't have a university/hospital where you can apply to do clinical research. So I recommend that if you truly enjoy research, find a long-term position on campus/hospital/med school. Even if it's just a few hours a week, you develop repertoire with the P.I (REC LETTER!!!) and steadily gain new knowledge/skills. Not to mention, this shows dedication.
3.) Clinical education programs:
-SHPEP
-Dialysis Clinic Inc Summer Internship
-Volunteer programs in another country for the long-term (don't do the programs that last like 2 weeks with only 3 days of clinical care)
-Many other types of "pre-med internships" you can find online
The "internships" are useful to get mentor-ship from med students and health personnel and help you create a vision for what you may want to do on your pre-med route. This might be a waste of time if you're pretty educated on pre-med life. You get tours of medical schools, hear speeches, etc. I haven't done this, but i get mixed reviews about it being helpful. You might find some like the dialysis clinic-- gems in which you actually get to partake in care, so search well!
Beware of volunteering abroad: Not many SDNers like to support voluntourism because you don't really make a difference if you're there for a short period. You'll just be criticized for taking a glorified vacation. However, see if you can find a successfully sustained program that will allow you to stay for long periods of time. You can see over time how your interactions have helped a community in the long run.
In summary, I think continuing the long-term activities (EMT, hospital volunteering, non-clinical volunteering) you do during the school year into summer will still be AWESOME for your application because it displays your dedication. Don't be pressured to have "special" summers, but these options can spice up your activities' variety if you truly find potential value in them. Long-term commitments will almost always trump short ones (unless you plan to do something like the camp for a week every year). Also, with these summer programs, you might risk not being able to take a summer class or continue that job. There's lots of ways of moving forward as a pre-med without traveling an inch, okay!