http://www.Thank-You-Note-Samples.com/business.html

Lots of sample thank you notes for business and personal topics such as meetings, interviews, wedding gifts, etc. Or you can fill out the form at "I'll Write It" and get a custom-written note based on the info you enter.

Here's a short sample but there are longer ones on the website too:

Dear Todd,

It was a pleasure meeting you yesterday. Your ideas on new approaches to sales in our region were insightful and a great help to me. Thank you for making time to meet with me and for sharing your thoughts.

Sincerely,

Synthetic biology or wetware concepts. Nanotechnology, using biological pathways repurposed as molecular fabricators. Solar collection arrays that use photosynthetic reaction centers to convert light to chemical energy. Mechanical energy storage, in bio-molecular actin springs.

http://www.thenewatlantis.com/publications/the-promise-and-perils-of-synthetic-biology

Some ideas:

Restaurant, typically runs different weekend staff.
Retail, weekends are busier in this industry.
Service station, open around the clock.
Hospital custodian, they are always open.
Student painter, window washing.

You probably will have a hard time finding any employer that will be able to accommodate extended absences like your summer and winter break, but being up front about it will give you a better chance of being rehired that if you don't.

As for the painting and window washing, I knew people who started a small business doing this and they did ok during school. Set their own hours, taking only the work they has time for. With reasonable prices and a reputation, you should be able to do well.