One of the best things to do (and also one of the hardest to properly execute), in my opinion, is to give the player the illusion of choice without very much actually existing. But more specifically, this illusion should be one that is not easily dispelled, and this is the hard part. When giving freedom to the player, the developer is essentially relinquishing some of their own freedom. Certain things cannot be put into the game when the scope is too large because it would be impossible to account for anything. As Bethesda games are the quintessential sandbox games of the last decade, I could cite an example from there. A good portion of the Elder Scrolls and Fallout environments sort of just...exist with no purpose. There are invisible walls, there are places that you can't reach even if you wanted to. This is usually true for games, though, save perhaps Assassin's Creed. However, if you look at something like Splinter Cell: Conviction, which is more or less a straight line, you are offered more interesting ways of using your environment. You can climb certain pipes and hang off ledges, etc. but this all gets repetitive. Some sandbox games do the same things as SC:C, by adding certain gimmicks (with your example of Just Cause, there's the parachute), but they fall into the same trap of repetition. Providing a variety of gimmicks, ones that change with the situation and constantly give the player something new to experiment with creates this "illusion of choice," allowing for developers to add cool-looking sequences (Divinity 2's combat looks pretty cool, but only for the first couple of enemies) that don't outstay their welcome. However, in order to make this work, it is also necessary to make sure that the learning curve isn't too steep in each scenario, else the player will simply ignore the mechanics that were implemented in order to make the experience more fun in the first place.