We did not stay in Medellin as we are not big fans of cities, but if you are short on time, and you can only spare a few hours, make sure to visit Comuna 13, you will understand a bit better the struggles of this city.
I will actually not put too much text into it, as you can read a lot of this info online.
But what i would actually recommended, is to book a tour with someone local to bring you around and explain you their experience over the years.
We booked our free tour via GuruWalk and felt it was worth our time and money :)
Maybe 1 note, if you love streetart -- this place is a must must must, but you probably already know this
Make sure to go up the stairs, on one hand you will understand how complicated it is to leave up (imagine having to Cary a washing machine) and how much the electric stairs that cover part of the climb help) and on the other hand you get this view: