Waterproof Jacket and hat!
Miscellaneous: If you are interested in the four WW1 battles and want to visit Ypres/Ieper and the Salient, a "must have" is Major & Mrs Holt's Ypres Salient book (isbn 0 85052 551 9). It's about £15 but worth it and it comes with a map marked with all the "sights"
Good walking shoes are so important. Its cobblestones, cobblestones and cobblestones. Everywhere.
Of course you are in Belgium and wet weather gear maybe needed.
Photo Equipment: Bring plenty of film, or memory cards for the digital.