I just spent a month in Colombia, and three weeks of that was on the Colombian coast. My boyfriend is from there. I would agree with Cartagena if you have a young child. It was packed with Argentine and Colombian tourists when I was there (as was the whole coast), but if you're just now in the planning stages that won't be a problem for you.
The historical center is absolutely magical. If you can stay there, do. I found nothing dumpy about the part outside of the center, but we stayed in Bocagrande. Lots of new high rise buildings there. Cartagena can be as upscale as you are willing to pay for. The beaches are nice, much better than Argentina and Uruguay, but the swimming pool-blue water can only be found if you take a boat out to Rosario or Playa Blanca. You will have to say no, gracias about 50 times per day as the people selling things on the beach are quite aggressive. Not at all like in Brazil. Be especially aware of the women selling massages. They are the most annoying of them all... Even in the water the guy selling ice cream was swimming around with the cooler trying to sell it!! But oh, lunch on the beach is a delight. Pescado frito, arroz con coco, patacones, ensalada. I already miss it.
If you have any specific questions, feel free to PM me.