Hmm, this is an interesting one, I would firstly have a conversation with them to find out why they are handing them in so early, it may be that they have other commitments (Such as work, family or other study) that means they must be super organised, or maybe they just finding the work too easy and not challenging enough for them. They also may not want to do anything extra for the course, especially if they are already committed in other areas. Also, is the work to a high standard? If it is, and they do want to extend themselves with further work, you could get them to run a couple of share sessions with other students about what they have learnt on the course. Another option is they may want to offer to help support other students that may be struggling. I would also ensure there is some sort of reward for them doing this, either in the form of extra credits, or an excellence certificate or similar. Hope this helps!