It's very cheap to travel in Medellín so don't limit your activities to that area. The train costs only 2mil COP which is about $1 USD and will take you everywhere. I stayed in Parque Lleras and Poblado and Parque is definitely a nice spot. I don't know much about the other areas but I have taken the train plenty of times near the Universities and other Parques near Centro, etc. It is relatively safe during the day and I never had any incidents but at night once when I was walking with a Colombian amiga, 2 guys asked for the time so that I could pull out my phone and they could steal it. My Colombiana pulled me away immediately to avoid them.
So yeah, things are around but be smart of where you're going
So yeah, things are around but be smart of where you're going