Is doing an unpaid intership during summer instead of a paying job a good idea?
The most important thing for you to consider, if you do not have to make the money, is what benefit the unpaid experience would have on the attainment of your career goal. The experience gained and the networking opportunities gained and the opportunity to see the inside view of that career area, would be very valuable in determining the proper career path for you pursue. These are very valuable and difficult to quantify,
Best of luck! Be true to yourself. The feeling and concept of success is yours - and is very personal. Let me know if this is of help.
If you can afford it, the unpaid internship in your desired field will be more beneficial in the long run. Many colleges are now requiring an internship as a condition of graduating. If money is an issue, you can always get a part-time job to provide some income during this time.
If you do not need the money, I believe internship is the way to go. The "hands-on" experience you will get in your career field is priceless and will give you an advantage over others competing for similar positions with no experience. In addition, oftentimes, the company you intern with can turn into a fulltime, paying position.
Good luck to you!!
I agree that, money notwithstanding, the internship in your field would be more beneficial - unless, of course, the job is, too. Doing something in your field gives you both the practical experience as well as the opportunity to network, get a better understanding of the field in a real setting, and may evolve into a future position.