Ideally, I think the best gift that you could possibly give a CS student would come from your start-up, but I don't think it's what you have in mind. The best thing that you could give a CS student would be a job working for your start-up.

That being said, if the app is something that targets a CS student's interests, yet isn't something that they are already likely to have, then a free subscription / license would be the next best thing.