Visible crew/equipment: Right as Young Biff starts to chase after Marty through the tunnel, there is a shot of the wheel spinning around, and you can see the camera crew reflected in the hubcap. (01:29:45)
Visible crew/equipment: At the end of the famous court yard chase scene when Griff and gang crash through the courthouse windows, you can see in plain sight the reflection of cables and the mechanisms that held them in the air. [Corrected in 25th Anniversary edition.] (00:20:55)
Visible crew/equipment: When Old Biff says, "Something very familiar about all this," two crew-members are visible on the left side of Biff behind him. (00:19:10)
Visible crew/equipment: If you look at the money case that Doc brings with him that has all the different types of money for each year, you will see where it says 1864 the reflection on the glass or plastic of what appears to be a tripod most likely for lighting. (01:03:47)
Visible crew/equipment: Just as Griff is about to crash through the front door of the court house after he is chasing Marty on the hover board, you can see someone (the director?) on some sort of crane in the reflection of the glass. (00:20:58)
Visible crew/equipment: When Doc, Marty and Jennifer are going to leave 1985 (at the beginning of the movie), the camera is reflected in the back part of the car.
Visible crew/equipment: After Marty runs away from what used to be his house in the alternate 1985, he walks towards a crime scene. There are two instances in this scene where you see the shadows of the camera and camera operator passing by on Marty's t-shirt.
Visible crew/equipment: When Marty realises that the almanac is not inside the dust jacket, a set light can be seen on the far right side of the screen. It can also be seen when he realises where Biff is. (01:21:08 - 01:21:45)
Answer: He is attempting to play the song "The Power Of Love" by Huey Lewis and The News. He was first seen playing it very well in the first Back To The Future movie when he was trying out for the school dance. Jennifer recognizes the song and is shocked that Marty who was so good at guitar was suddenly able to hardly play it.