What subject should I teach?

Major in the subject that you are deeply passionate about. I'm sure you have noticed this, but the best (and usually highest paid) science teachers are the ones who are passionate about what they teach the most. My best teachers (of various subjects) have always been those who really love their subject and never seem to be bothered to further help their students understand. If you are really worried about getting a good salary and if there are enough positions open, you also have to know that usually varies from school district to district.

If you still really want to double major, you can and to me that is never a bad idea as long as you know they can both be useful, but you also have to make sure that there is enough overlap to finish in about 4 to 4.5 years since you did mentioned that you don't want to waste money or fall behind.

Your other major should be the subject you want to teach in high school. If you hate grading papers, avoid English. If you don't like Chemistry, avoid all Science majors since one year of that course is usually required for all Science majors. However, don't let the large number of Biology majors dissuade you. Most are majoring in Biology because they want to go to Med School, not because they want to teach. Pick the major you want to spend your life teaching. :) With the No Child Left Behind Act, teachers who teach outside their major may not be hired in some school districts, or else not be paid as much. By majoring in the subject you teach, you will be more likely to get a job.

Second languages are a good idea. You can use it to teach ESL abroad too or maybe even become a translator. See this for other jobs where Spanish can be applicable, http://www.payscale.com/research/US/Skill=Spanish_... .

How about matyh? Otherwise, Spanish seems good.