Thor's choice of costume was, he thought, a pretty obvious one. It wasn't the first time he'd used the costume, but with the newest movie coming out, it seemed topical. So Thor was dressed as, well, Thor. It was the movie costume, with the addition of the comic book helmet, because helmets were just cool. Thor moved through the crowd, occasionally bopping people on the shoulder or the head with his inflatable Mjolnir reproduction.
When he saw the Black Knight, he planted himself right in front of him, held up the blow-up hammer like it was a sword, and boomed out, "I have no quarrel with you, good Sir Knight! But I must cross this bridge!"