My DS (fifth of six) will be 5 next month. He just discovered that, of all things, he loves needle felting!

Quick rocket kits...baking soda and vinegar, etc.
Other can't-miss things: Knights, castles, dragons in any shape or size.
Puppets and a doorway puppet theatre.
Costumes/dress-up!!
Books on CD. Someone already recommended Jim Weiss, and I can't second that enough. We have fifteen of his CDs. The soon-to-be-five-year-old is entranced by Galileo because of Jim Weiss' retellings of the story of Galileo's life!
Can he/they read alone? If so, Magic Tree House books and Magic School Bus books.
Puzzles! My 4yo is deeply in love with his big (BIG) puzzles...the ones with huge pieces that end up being 3 feet tall when you've put them together. Buzz Lightyear, Scooby, Dinosaurs, the Solar system...he has quite a few.

UNO!
Stratego...even more awesome since they can play each other. My guys love the LOTR Stratego, but they have LOTR nutty older sisters.

The Hobbit and The Hobbit on CD.
love, penelope