I just got back from Colombia last week. Spent 3 days in Bogota, 3 days in Cartagena, and 3 days in Medellin.
Bogota - We stayed in Zona T. It's a little more upscale, but still walkable and we felt safe where ever we went. We did one of the free walking tours which took us through downtown and the Candelaria district. Took about 3 hours and hit some popular spots like the main square. We ate at a restaurant called Leo which was on of the most unique places I've eaten at before. Not cheap, but if you had that meal in the US it would be way more expensive.
Cartagena - Super touristy. Anywhere you go inside the walls, people will try and sell you something every 10 feet. There's a bar that sits on top of the wall overlooking the water that everyone flocks to during sunset to get the Instagram pics. Glad we went, but I wouldn't go back to Cartagena again.
Medellin - By far my favorite place we visited. The views, the weather and the people are amazing. The airport is about an hour from the city due to it being on the other side of the mountain, but they are building a tunnel that will cut the time to about 20 minutes. Parque lleras is where everyone goes out at night, it's just a ton of bars and restaurants. Gusto is the club you want to go to if you want to see some of the hottest women in your life. But beware, every single one of those women is a hooker. We stayed at the Los Patios hostel and it was a really nice hostel. Rooftop bar with a great view, plenty of amenities, and a good place to meet other people. Make sure you do the communa 13 tour, it's amazing. It's incredible to see the transformation from the hood to what the neighborhood is now.
As always when traveling, just be aware of your surroundings while walking around the don't make yourself a target. We had a couple in one of the walking tours and the husband was wearing a chain. Someone walked up behind him in the middle of the shopping area and cut the chain off with a knife. Besides that, no issues with safety at all.
Make sure you brush up on your Spanish as well. Most of the locals don't speak English much, if at all.
Overall, I loved Colombia and will definitely be going back one day. Don't let people scare you away with safety concerns. Be smart, and enjoy the cheap, plentiful food and everything else the country has to offer.