Mathematical creativity: better a high profile or a "laid back" department?
Let me focus on the question in the title.  In my experience, a vibrant mathematical community is the best environment for creativity.  More prestigious universities/departments tend to attract creative and passionate people, who create vibrant communities, and being surrounded by them will help you be more creative and passionate.  While this is not a rule, and some departments just have bad atmospheres (you need some individual knowledge of the department to know about this, so visits etc are good), you're more likely to find inspiration at a place with cutting-edge research, than at a place where most of the research is "pedestrian."

There is a caveat, however.  If you can barely keep you head above water, it's hard to be creative.  Fortunately, most places won't admit you if they don't think you can handle it, though it does happen.  Again a visit, or browsing student/faculty webpages/blogs may help with this. See also this related question on MSE.