My niece's marching band played at Walt Disney World over Thanksgiving weekend last year. I didn't find it that bad. I was at the Magic Kingdom on Thanksgiving day and following Saturday and Animal Kingdom on Black Friday. The only day that approached being as busy as I expected was on the Saturday. Even then, it wasn't like you could only get on the rides you had FastPasses for. As long as you mentally prepare for the bigger crowd, you should be okay