We spent a lot of time mucking about shooting tigers and getting up to other shenanigans, but we also knocked out a solid chunk of the main story missions. Each one seemed to take us to another unexplored portion of the map (which is huge, by the way), and every time we headed to uncharted territory we found ourselves climbing another radio tower.
After the first one, you're not explicitly told to do so, but you will anyway; it's extremely rewarding. Climbing up isn't too much of a challenge, but the view once you get there is breathtaking. And by destroying the pirates' jammers, another square of your map will be filled in, and you can see everything from nearby objectives to what areas are good for hunting boar at that particular moment. Then ziplining back down is a blast, of course.