Last summer, I made most of my money (and come to think of it, quite a bit of money) as a Musician. As a musician, my income came from 4 sources:
1) Performance as a member of Aces High, a blues band. The band's take would range from $300 and perks for a smaller party, to over $2,000 for some of the larger Midwestern blues festivals. The group was fairly large, and we had some non-musicians on the payroll, but my take was still typically over 40/hour. For each show we played (typically 60-120 min sets), there was 4 times that much time advertising and promoting.
2) Sales from merchandise. We sold CDs, T-shirts and Stickers, promoted them, and divided up the loot. This was not a very large revenue source, but it was nothing to sneeze at.
3) teaching, both individuals, and groups. Nearly every member of the group taught private lessons. I had 4 students over the summer that I gave private drum lessons for, and then 2 school districts where I taught a clinic on the finer parts of Marching Percussion. Private lessons = great work if you can get it (22/hr), and clinician work was even better, although not very steady.
4) Consulting. We became the most successful "young band" to come out of our corner in a long time. Other musicians sometimes paid us to help promote them, design logos or ads (we had a graphic designer in the band), and get their house in order. Again, this wasn't very steady work, but the pay was pretty good.
In addition to all of this, I also had a straight job where I worked around 15-20 hours a week, because work as a musician isn't always steady. I might clear 1,000 in a week, and then only get 45 the next. I did it because I love making music, and "the perks" that come with being a musician, and I loved the flexibility and adventures that came with it.
It was a LOT of work though. I was busy either practicing, performing, or preparing most nights and weekends during the summer. Every show has hours and hours of backwork that goes into it. I learned a lot, and had a lot of fun though! Depending on how flexible my government job is this summer, I might try it again.