My boys do all the typical stuff (legos, playdough...). Their favorite past-time is building with the couch cushions.
Today, of all things, they've decided to start sewing. I guess after watching their sisters make things they wanted to try it to. My 5 year old made a pillow (sort of). And the 7 year old is cutting out shapes and sewing them onto a piece of fabric. The 4 yr old just liked the thrill of handling a needle and sharp scissors. (Oh, and he also enjoyed cutting fabric into millions of tiny bits.)
An idea I haven't used yet, but sounded nice, was to fill a large container about 1/4 of the way with macaroni noodles and let them play in that with trucks, spoons, etc... I've done this with sand in the past & it was hard to clean up. The noodles would be easier.