I'd like to put a word in for the early morning walk. Before the streets are crowded and all business is happening. There will still be a interesting activity to observe as the city wakes up and begins preparation for the new day. Maybe you'll find out where the cabbies meet and what they do to pass the slower hours. Who buys the first bread when the bakeries open. In some cities, there may be animals about that would wouldn't see during the day!