I hope they have a great time. Colombia is much safer now than it was in the past. Like most other places in the world, you won't find trouble unless you look for it. Medellin is a relatively modern city and has made strides in its infrastructure after some successful technocratic mayors. I'd only suggest extra vigilance if they plan on partying throughout the night in Medellin. Obviously don't go looking for drugs or follow strange women around, and don't wander off drunk and alone. The countryside will be beautiful for them. Perfect temperature and the mountains around Medellin hold some of the largest flower farms in the world. I think only second behind the Netherlands. I would highly recommend touring some of the farms if they can, or taking some horseback rides like caulfield mentioned. It is truly gorgeous. Many people recommend a tour of Guatape and climbing the Guatape rock is a big tourist activity. It is pretty, but I also found it to be a pain in the ass getting up it, especially on a hot day. Should be mindful that it's a taxing hike if anyone has mobility issues. Lastly, they should gorge themselves on empanadas. They'll salivate the rest of their lives thinking about them 😁