Volunteer in Teaching
Teaching English as a foreign language is a popular gap year option and volunteering as a teacher is a great way to gain classroom experience first-hand.
If you're thinking about teaching as a career, TEFL is a perfect starting point - you can make a career of TEFL in itself! Also, it's one of the best ways to further fund your gap year. Travel, teach, travel. Easy.
There are all sorts of ways to volunteer as a teacher and there are huge benefits to the students. Often, learning English is one of the best routes out of poverty for kids in much of the world and even learning a few words can help them later on in life, so if you think you've got what it takes to teach English as a foreign language then jump in a the deep-end. It's not something you'll regret.