Cartagena is great, its always nice to watch the sunset from the walls and enjoy the temperature dropping down to more tolerable levels.

If you want to check out a really nice colonial town that is way less touristy you should try to make it to Mompox. Not only is the colonial center in great condition but there is also a pack of howler monkeys that roam through the town. Its a little bit isolated but well worth the trip and isn't too hard to reach from Cartagena.