When I was an undergrad (about 2 years ago), I'd teach guitar. $30 for a half hour lesson. I had close to 20 students at any one time and would rake in roughly $500 a week with that gig. That ends up being around $24,000/year. Plus it was an easy way to meet pretty girls :-)

There are plenty of ideas out there. Just figure out what all of your goals are and try to align them into one gig.