IMO, pretty important, for 2 reasons:
1) it's important to have an outlet, both in school and after. medicine is great, but well-rounded doctors are happy, competent doctors.
2) as most of us found, having interesting stuff on your resume stands out and makes you different. everyone has grades, step, etc. most have pubs. but if you were a triathlete, or mentored kids, or traveled, or whatever, you make an impression.
as a 1st yr student, though, the biggest advice i would give--despite the bashing i may incur-- is rock step 1. in my (extensive, personal) experience, that is the hardest black mark to recover from.
cheers.
firegirl