Oh, and I think that there are some characters at that time too(from what I hear - I've never actually been there early, but we will be on our Oct. trip!!!)
I have always seen characters on Main Street when the park opens early. Usually quite a good variety, mainly in the area directly in front of the train station.
We ran into both Mickey and Minnie on Main Street before the park opened. It was great getting that done before the day really started. That was on second day at the park. The first day, my DD was yelling out Mickey's name as we walked in, expecting him to be right there waiting for her. The second day he was, and it was a real magical moment.