One of my favorite things too when I am in Paris is to watch the light change on the Seine. From early morning light to the light just before the sun sets. Coming to Ile St Louis to watch the sunset and the lights turn on was on my list. Thankfully I did it at the beginning of October when it was somewhat warm once the sun set. If you have the chance I highly recommend it. I made sure to find this spot several times during my month in Paris.