I can only speak for single player, but I'm sure others can chime in about pvp.
In single player there is a LOT to do:
1. Kingslayer files all over the map, in buildings, outhouses etc to collect and read documentation about the people or find audio logs.
2. Side missions include: Stealing a plane and delivering it, stealing a helicopter before the enemy flies off and you fail it, finding and destroying convoy trucks (armored transport) and then tagging the mentioned truck for a reward and hacking into a station in a given time frame while fending off enemies.
3. A wide away of vehicles for land, air and sea. From dirt bikes to vans, people carriers to sports cars and buggies on the land. Planes, helicopters, apaches, fishing boats, tour boats etc.
4. Huge outposts, prisons, camps to take over either stealthily in the bushes with a sniper or by force with a mounted gun.
5.Last but not least.. Unidad. The elite force of the island which are purple on the map and they come in hard and fast with no remorse. Imagine the army on GTA games.
It's third person mainly but aiming down sites is first person.