Walking back from dinner on our first night in Paris, we were stopped at a crosswalk for a long time. After a few minutes, we see tons of rollerbladers coming down the street. We had heard of this before; every Friday in front of the Mayor's residence basically everyone groups up to rollerblade through Paris.
It was pretty cool, but I don't think we would have been able to keep up - they were going pretty fast. But we couldn't have timed it more perfectly if we tried.