Probably when I studied abroad in London. I got to live in a great part of the city in a huge house with other students and an amazing professor, and classes were held in the house's library (no classes on Friday). We also went to a play every week.

I visited so many cool places with my friends on weekends and holidays. The other people in my group were also really into museums and eating new things, so each place we visited was a learning experience. I also got to show my group around Vienna, which is my favorite vacation destination and spiritual home.

Maybe not a life-changing experience in the way you are probably asking, but an excessive and amazing experience nonetheless.