I imagine Bogota would be a natural place to pass through and visit as well. Although not sooo interested in the cities people say that Cali has great salsa dancing (which I love) and would also love to see all the Botero works in Medellin.
I would be traveling to and from Quito on this trip and would be willing to do overnight buses (where not completely sketchy) and internal flights to make things go faster.
Is there anything I am missing? Also, if I only had a maximum of 2-3 weeks, what can I reasonably see in that amount of time and what SHOULD I make sure I don't miss?
Cartagena is hard to beat for history. Look it up in the encyclopedia. It has massive fortifications beyond anything I'd suspect you could find elsewhere in the Americas, and a grisly history of killing hundreds of thousands of black slaves in their construction. It was the center from which the Spanish galleons were loaded with gold and silver from Peru and elsewhere to be sent to Spain....a fascinating place to this day. I met a beautiful young black woman on the beach there who simply wanted to practice her English. She told me many things in a long morning conversation about Columbian society and greatly impressed me with her intelligence, wisdom and knowledge. I was deeply impressed. She offered to take my children on her own on a tour, so I insisted on giving her money to facilitate that. It was an extraordinary response. She didn't want the money but reluctantly submitted to my insistence. She ended up spending it to purchase gifts for my kids when I wasn't around. My son and daughter had a wonderful time. I've met no ambassador more gracious in all my travels.
On the other hand there is grueling poverty in proximity to prosperity. A leper with a hole in his face where his nose used to be tapped me on the shoulder from behind and had the remains of his hand extended to me in a gesture of begging that was clearly calculated to shock. I shall always remember that too. I think he was pleased, whether by my shock or donation or both, I shall never know.