Sometimes the simplest things can bring about the most fun. Just Cause 2, while giving you a lengthy campaign of missions to play through, excels primarily because it gives you the tools required to have a good time - a rather explosive one at that. Avalanche's open-world sequel is silly, doesn't handle gunplay all that well and features the worst voice acting I've heard in a very long time, but once you learn the ins and outs of Just Cause 2's grappling hook, it's hard to stop messing about.
Don't buy Just Cause 2 expecting the kind of involved story you found in GTA4. Playing once again as secret agent Rico Rodriguez, you're sent to the islands of Panau to hunt down a rogue agent. That's the basic plot, but soon things become much more than Rico had bargained for, so you'll need the help of Panau's factions if you're going to survive against a trigger happy military regime.
Structurally the game is similar to Saints Row 2, in that story progression missions are only unlocked once you've completed a number of side-missions and racked up Chaos points. Chaos in Just Cause 2 is handed out for completing missions and for destroying government property, with the game informing you how close you are to unlocking the next mission every time your overall total is increased.
Completing faction missions not only rewards you with Chaos points, in turn getting you close to the next Agency story mission, but also widens that faction's area of influence. These factions will provide you with support should you run into trouble, as long as their influence stretches into the area you're fighting, so despite appearing as side missions they are essential for your safe passage throughout the game.
At times this system results in an island map completely devoid of missions, leaving only the cash-rewarding race events to take part in. As these don't hand out Chaos points, you're left to cause as much damage to government property as possible until you cross the required points threshold to unlock a new mission. It's a tad tedious at times, but the wanton destruction is usually great fun. Just be aware that you won't be able to focus purely on working through the story.
Mission structure then isn't going to be ideal for everyone, but something that few will be disappointed with is the insane stunts Rico can perform. These are built around his ability to skydive, his parachute and grappling hook. Just Cause 2 isn't a realistic game, and as such can get away with some insane moves. The thrill of skydiving out of a plane thousands of feet in the air, then ripping the pull chord on your parachute inches from the ground is just brilliant, while being able to tether yourself to all objects means you can traverse large distances with a neat grappling hook/parachute combo.
There's always a normal way to climb up to the top floor of a building, but why bother when you can just grapple up the side? Why shoot an enemy when you could pull him from his perch with a click of a button using your grappling hook? Is a pursuing enemy vehicle causing you trouble? No problem. Just tether the Jeep to the road and watch it spiral into the air as if from a Hollywood action movie. Why not attach a solider to a gas cylinder and watch what happens when you pop a cap in the tube. These are just a fraction of the ways you can have fun with the tools in Just Cause 2, and the reason the game is so much fun.