Has to be my school trip to London (second time in London) for New Year's 2012. My school's band and choir were invited to play/sing/march as part of the queen's Diamond Jubilee celebration. Warwick Castle, Stratford-upon-Avon (birthplace of Shakespeare), shopping places in London, Nando's (lolz), had a nice dance in the hotel's ballroom with other schools from the US as our NYE celebration, and other stuff I can't remember.
It was so memorable it hurts. Wish I could do it again, but performing in such a big choir, marching in the streets of London in drenching rain, just everything would be impossible to duplicate. Made good friends with a lot of people.