If you ask me story mode is interesting and it also helps you achieve more stuff. For example you unlock new skills, items and more.
Story mode is also fun, but to make it more fun you can compete against your friends, speed run it or try to record video or stream it to your friends or on some platforms.

If you are a guy who enjoys defeating enemies its your thing to do.
Try to have a big chase, steal their vehicles and use it against them, for example helicopters, tanks.
Fighting against them can also help you. You can find useful weapon for you which can be used anywhere.

As we know, "Just Cause" is well-known for It's physics and endless possibilities. You can do so much stuff like destroying bridges, grappling anywhere you like.

I enjoy destroying bridges while someone is crossing it. For example i grapple cars in the middle of the bridge and then shoot rocket launcher to the bridge and then it collapses.

Also you can shoot grapples with balloons (it can be unlocked after some missions) at car and then it will start going in the air. you can grapple people on the car or you can stand on the car and go in the air with it. you can also use that grapple to bind people together and they will start floating as well.

You can add some mods in the game like the vehicle pack in the images, Or you can install more mods to play with. Choices are big.

You can do some stunts and there are so many ways you could do that. You can either glide, use parachute, car or grapple to do it. Just think of a way to do it and it will always be possible to do it.
You can also re-create someone else's stunts.

If you are guy who likes mysteries or who is into adventures then try to find some interesting stuff. You can find many interesting stuff in the game and i wont spoil it.
Journey in this game never ends.

As i said before, there is much to explore in the game.
Why not make exploring the map better and more fun?
You can compete against your friend who can find better spot in the game to explore.
You can also set a timer and go around the map as fast as possible. Try to beat the world record of going around the map or compete against friends.

I think this is it. I don't have any more ideas. If you have some don't hesitate to type it in the comment section.