One answer is simpler than you might think: demonstrate your love of learning. Colleges are, after all, academic institutions, and want students who will engage intellectually with the resources at hand.
The key to this is the key to most things in life: passion. Colleges don’t want to see that you’ve joined every club and organization your high school offers; they want to see evidence of your passion for a couple of things. Colleges want to enrich you, but they also want you to enrich them, and people with passion are driven to include others in the pursuit of that passion.
In essence: Find something you love to do, and do it.