Getting a part-time job rarely gives international students new working skills, but always helps boost speaking and listening proficiency as well as cultural knowledge.
Many part-time jobs are easy to get, but don't have a large degree of customer contact. Perhaps the worst possible part-time job to do is as a cleaner - generally you will be by yourself because managers know that cleaners usually don't clean when they are with someone that they can talk to.
If you work as a kitchen hand you will talk to the cooks and the boss, but you may spend long periods of time just washing dishes with no chances to interact. Working in a supermarket stacking shelves could be similar, but the time spent talking to coworkers and managers.
Working in a bar is generally not the best plan, but works well for students that already have a high level of speaking confidence. Work in a bar is very tiring because while everyone else is able to drink to keep their energy levels high, you have to maintain your level of interest in the interactions despite feeling sleepier and sleepier. Bars are only open at night, and some are open into the early morning which will make it much harder to study well during the day.
Working in a fashion shop is a good choice if you are interested in clothes. You can have long conversations with some customers who will come from a variety of backgrounds. Usually these jobs require little experience and will accept international students easily. If you have a coworker then it will be even better. However, many shops don't have many customers, and you may find that you are sitting in a shop all day with no-one to talk to.
The best kind of job is working as a waiter/waitress in either a cafe or a restaurant. You have to have a lot of short conversations, but usually there are quieter times when you can talk more to coworkers and customers who aren't in a rush. Just make sure that you don't work in a restaurant that sells food from your own country. This will pay close to nothing and you will speak very little English. The only advantage is that it is easy to get these jobs. That is because people quit quickly.