Sorry this thread is a bit old but it's not too late to post a reply. You're certainly lucky to be able to have the opportunity to travel for a gap year, just make sure that you have every detail that can be planned, planned. I think that's the main tip to remember before going overseas as an 18/19 year old.
On the question of whether Hawaii or Japan, I personally would visit Hawaii just for the relaxation purposes after a long year. If you're a really independent person who enjoys the outdoors I think Hawaii is good - you've probably looked at alot of brochures but there is one here that is gives a good overview of the islands and what they offer:
Hawaii Brochure
The only negative I think is not going with anyone else, Hawaii would probably best be enjoyed with friends since depending on what you're doing it seems to be alot more adventurous in activity.
Japan sounds more individualistic and rich in cultural heritage, and much more congested. I think this would be better for you if you plan to travel alone, but you would need to organise things like english translation if you aren't planning on joining a tour group. This is probably a small matter if you join a group.
It really bubbles down to whatever you're trying to get out of your trip, either way it's going to be different to your everyday. Have a great time!