1) When Arthur Weasley leads Harry and his friends to the Ministry of Magic, they must first dial the secret code on the telephone panel. He enters the numbers 62442. The letters under these numbers on the standard mobile phone keyboard give the word magic, "magic."
2) If the Muggle saw Hogwarts, then before him would appear old ruins with the inscription: "Do not enter, dangerous structure."
3) Sirius Black got his name from the name of the brightest star in the night sky, also called the Dog Star. It's very wise, since Cyrus, animagus, is reincarnated as a big black dog.