Puzzles and books are good options. Also, blocks or lego duplos so he can build and be creative, but it is something he can do sitting down and not running around.
Also, I sell Discovery Toys and we have a really neat game for three year olds that could keep him engaged for a long stretch of time. It is called "It's A Match" and you can buy the basic set, with two books, or the deluxe set, with four books. View it at http://www.discoverytoyslink.com/karenchao My four year old really enjoys it (and liked it when he was three, too).
Another great Discovery Toy is Playful Patterns, where he can make different mosaics with foam shapes. It is perfect for a preschool-aged child, because it helps form the basis for math and reading, plus helps build their thinking skills and fine motor skills.
The last one I'd like to recommend is actually our number one selling toy in the entire catalog - MotorWorks. It's a set with a car, motorcycle, and airplane that you can put together with a battery-powered screwdriver, then take them apart again. It's a lot of fun. I recently gave it to one of my son's friends and he played with it almost non-stop for three hours (pausing only for a snack).
K.
http://www.discoverytoyslink.com/karenchao (summer sale thru 8/21 includes Playful Patterns at 20% off!)