I spent about 3 or 4 months in Colombia when I was traveling. I think I spent close to two months here.
Cartehenga is a good place to start, but I would head directly to Taganga if I were you.
I enjoyed Bogota as well, but you have to be a bit more careful there... depending on where you stay. I traveled through Colombia to the Venezuelan border, spend a couple of months in Venezuela, and then back through Colombia to the Ecuador border and spend a three months there (two months in the Galapagos).
Anyway, try to keep bus travel to the days if possible. I probably wouldn't travel by but to Ecuador again, that was one of the more sketchy trips.
Buses are kept really, really cold. Bring a blanket!
It is a beautiful country if you have the time to spend.
If you are looking to party, go to Cali.