Based on keeping up with your recent Central American trek, it's going to be hard to decide and limit where not to go! Since you're starting/ending in Bogota, the trampled route heads up through Bucaramanga towards the coast and Cartagena, and then sometimes back through Medellin or straight back to Bogota. Trouble is...there are SO MANY jumping-off points to stop along the way.
If you got that price on the flight, book it while it's hot and you'll have 6 months to come up with a plan! From reading the latest reviews, the new LP Colombia guidebook or other series' recent editions aren't much better than the older ones. You'll find loads of info right here on TT in the Colombia category, and numerous active posters that can give you timely and accurate knowledge on specifics on/off the tourist trail. My month last year barely scratched the Caribbean coast but based on your apparent interest in history, Cartagena for two days in the city and another day to take the day-trip to Volcán Totuomo.
Wasn't overly impressed with Playa Blanca/Islas Rosario. Head to Parque Nacional Tayrona for beach, and a lot of the younger crowd seems to enjoy Taganga though I much preferred Santa Marta - even more so than Cartagena. And if you're a Simón Bolivar or Gabriel García Márquez fan, can definitely hook you up along the coast. (Supposedly the coast gets crowded during Christmas; might plan for that.)
If you want to stop through Medellin, would be nice to finally meet you. Medellin has a fantastic Luces de Navidad celebration to enjoy for the holidays. Will probably do some reading-up on Pablo Escobar and go skulkin' around with lite research, want to do the hang-gliding thing and take at least a day in Santa Fe de Antoquia, but otherwise just hang and chill on a real casual local level. Suerte chica; compared to México, Guatemala and Honduras, you're going to love Colombia!
The More I Go...The Less I Know ~
2019: Puerto Rico, Costa Rica, El Salvador, NYC, Croatia, Montenegro, Bosnia & Herzegovina, Istanbul, American Southwest Grand Circle, Los Angeles, Brazil/Peru Amazonas, Colombia.
The Ozarks and Buffalo National River area keep me occupied while home.