Charlie will send you on survival missions with waves of enemies, which allows you to build up quite a bankroll if you’re good enough. Bringing a buddy will be a great help.
Grim will send you on stealth missions, where the better a Ghost you are, the higher your score and the better your payoff.
Briggs will give you co-op missions. You’ll need a friend to take them on at all.
Kobin will give you missions to wipe out terrorist bases, such as an opium operation. To avoid lots of reinforcements, you’ll need to be subtle.
All of these missions have lots of nooks and crannies to explore and more than one way to succeed. And the money really comes in handy. Do keep in mind that the missions get progressively harder and you will need certain pieces of equipment to prevent them from being cripplingly challenging as you progress.