Fab, even though I gave a shitty review of Santa Marta, I wouldn't write off the Caribbean coast of Colombia entirely, there's good times to be had there, I just wouldn't stay in one place for an entire month.
If I had to do the coast again (which I may in the future as I did it ass backwards last time) I would do it like this:
Cartagena: 2-3 days to check out the old city, its nice. Then I'd take a boat from there to Playa Blanca (about 25K), I was only there for an afternoon but its a really nice, white sand beach (unlike those in Cartagena). I ran into a few back backers there and you can rent hammocks on the beach for 10K/night, its rustic but laid back and chill.
From Cartagena I'd then go to Santa Marta for a couple of days, like I said in your other thread, nothing special but worth a visit. I stayed at Dreamer's hostel, a nice place but a bit over priced (75K, no a/c) and far from the beach, then I stayed at hotel Caribe which is downtown near the beach (and has a/c!).
You could also check out Minca for a day or two, its a small town in the Sierra Nevada mountains overlooking Santa Marta, I spent a day there doing some hiking and bird watching, which was actually a lot funner than it sounds. Again, its rustic but you can rent some cool cabins up there.
Then I'd just hang out in Tagnonga/Tayrona for a couple of weeks, maybe do some diving, hiking and lots of partying.
You could also hit up Barranquilla for a couple of days, nothing special there to see but it would be a decent place to find a Costena and take her with you up to Santa Marta area. Like all of Colombia, it shouldn't be too hard to scoop up a honey from one of the cities and take her with you to a tourist destination for a few days.
You can get to all of these cities using this company, its 20K from Quilla to Santa Marta and 40K from Cartagena to Santa Marta:
http://www.marsolsa.com/home.html