Books are always good, as so many kids have too many toys (or is it just mine?). My four year old has been a huge fan of his "The Complete Adventures of Curious George", as well as "Thomas The Tank Engine, the Complete Collection" and "Winnie-the-Pooh, the Complete Collection of Stories and Poems" since he was 3 or so. All are big books suitable for a big occasion, all have fantastic drawings and classic stories, and books are more easily shared than toys I think.
I have just used www.mywishwand.co.uk for my daughters 6th birthday, They gave me suggestions not just based on her age and gender but her interests too. We got great suggestions which we could buy immediately and it turned her birthday into a roaring success