Working at a restaurant or bar
Student occupations have traditionally been in bars and restaurants, whether you're propping one up or pouring drinks behind the bar. Student towns are never short of licensed establishments, and there's a good case to be made that if you're going to spend half your time in bars anyway, you may as well be paid for part of it. There are several benefits as well.
Working at a bar or restaurant may be enjoyable, especially if you are a "people person" who enjoys meeting new people. Working nights and weekends allows you to tailor your hours around your classes. It may also help you advance — one of my friends enjoyed his bartender job at university so much that he decided to make a profession out of it and became the UK's champion cocktail mixer.
Jobs in Retail
Part-time retail jobs while you're studying may not seem like a fancy choice or precisely what you went to university for, but they may be a wonderful opportunity to meet people while making a little money. I'm not talking about stacking shelves or flipping burgers (though neither is wrong).
If you're a music fanatic, film buff, book nerd, art-aholic (I'm out of words here, but you get the idea) or have a keen interest in any other marketable commodity (fashion, sports, food and drink, facial piercings, whatever), working a few hours per week in retail jobs that cater to those with your interests will earn you some pocket money and introduce you to likeminded people.
On-campus student employment include receptionist, librarian, and others.
Most university campuses will have some form of part-time student job available, whether it's a paid employment like receptionist, staffing the college bar, or an assistant librarianship, or volunteer obligations that earn you additional benefits like free housing.
Jobs as a private tutor
This one may be a winner, especially for graduate students or those in their third or fourth year of undergraduate study. Tutoring positions may be easier to get in larger cities, but they are the sort of employment – the golden grail! – where you can make money by using the information you earned from all of those hours spent reading esoteric books in a library or mixing up test tubes in a lab.
It may seem strange, but there are many students farther down the academic food chain whose parents would pay big money for access to your well-informed observations and recommendations on how you aced your school exams. Tutoring positions may be more difficult to get if you're an international student who attended a different school system, but they're a terrific way to make money if you're home for the holidays. Tutoring positions often pay higher hourly rates than other types of student work.
Volunteering
Doing a few hours of part-time employment every week might improve your university experience for reasons other than money. If you have a kind heart, consider volunteering for a charity, whether it's running the till at a charity store on a Sunday afternoon, assisting at a local homeless shelter, or spending a few hours every week fundraising.
Not only will this allow you to serve a good cause and meet others who share your beliefs, but it will also help you fill out your CV with some of the experience that is now practically required for obtaining job in the charity sector after you graduate.