A couple parts are funny:

1. When the twins yell to their mother that Sonny is fighting again with uncle Michael
2. When Fredo brings Michael to the room in Vegas and waltz in, he looks like Andy Kauffman.
3. When Fredo gets to the hotel with the 2 Million and the guy tries to take his bag and he is struggling for it.

Also Mary's frolicing scene after Michael tells her she can't see Vincent, freakin teletubbie

"Well you just do what I tell you to do! Goddamn it! If I had a wartime consiglieri -- a
Sicilian -- I wouldn't be in this shape! Pop had Genco -- look what I got." - Sonny Corleone