As life in Konstanz is not exactly cheap, you might want to earn a few extra bucks alongside studying:
The job board of the Student Service Seezeit is essential when looking for a suitable side job. In collaboration with the agency for employment they support you in finding a student job. Simply go to the job board on the web page, enter your search criteria and fill in the search mask.
The local newspaper Südkurier:
If you don’t feel like working in catering in the summer and at the Christmas market in winter you should have a look at the job adverts in the Südkurier. Here you will find countless offers beyond the seasonal jobs in the tourist industry. There should be something for every taste.
On the notice boards of the departments of the university and the HTWG you can find jobs that never make it as far as to the usual job placement services. It is more likely that you will find an interesting research assistant position than a temporary job as Santa Claus here, and that speaks for itself. You might also want to try the Career Service of the university.