My oldest son (almost 3) is one of those transportation kids, who loves anything to do with a vehicle (car/train/airplane). When he's inside, he does basically nothing but play with one of those three things. (He's also an outdoors kid, so this is only half to two thirds of the day usually, but still.)

While this makes shopping for him easier, we're concerned he's not experiencing a diverse enough range of things to learn effectively. He does read with us a lot, and if we specifically ask him if he'd like to do another activity (put together puzzles, color, playdough, etc.) he often will agree to participate, we can't necessarily always participate ourselves.

When he was in daycare, they seemed to handle this largely by defining the activity he may participate in; ie, "it is dress up time now", "coloring time", etc.; at any one time only one or two activities were presented as available. This certainly got him to participate in a diverse set of activities, and he seemed to enjoy them; but it seems rather draconian to us to schedule things that aggressively at home (and he's currently at home all the time now). 

We're proponents of the 'you should have fun as a kid' theory of parenting; at least until he's older we don't plan to push him hard academically, and so forcing him to not do what he'd like to do is somewhat abhorrent to us - but it also seems to be the only real option for encouraging him to do things to develop his mind in different ways. 

Is there a significant benefit to ensuring he participates in a wide range of activities at this age, or are we overthinking this, and should just let him play with his cars/trains and occasionally do other things with us, but allow him to spend most of his time doing what he wants? 

Also, if we're going to largely let him play as he wants but occasionally push other things when we're going to play with him, is there a particular set of things we should "supplement" with - playing with vehicles is heavy on the hand-eye coordination, both fine and gross motor control to some extent (the way he plays, anyway), and creativity/story telling; he also likes to organize vehicles and sometimes even count his vehicles.