The musical number with Stewie and a live-action Gene Kelly is an edited scene from the movie Anchors Aweigh (1945). The sequence originally involved Jerry Mouse of Tom and Jerry fame. Stewie's reflection on the floor remains that of Jerry.
When Stewie and Brian visit Gettysburg National Park there is an equestrian (rider mounted on a horse) statue visible in the background. That statue actually exists in the Gettysburg park. The statue is that of Confederate Lieutenant General James Longstreet (1821-1904) who commanded the First Army Corps during the battle of Gettysburg.