Magnetix. They come in all different size assortments. [17 pcs to 140 pcs!] The strong magnets allow kids to create shapes that can spin and twirl as well as stand or tower. Start with a small set and if your child goes crazy for them, as one of my did, you may be adding to their collection literally for years!
Jumbo Magneatos which are a similar product but supersized so that swallowing is not a hazard.
Zoobs. These are snap together parts that work on a ball-joint principle. Each kit comes with directions for assembling the zoobs into a variety of creatures [from turtles to chickens to raptors] and objects [chairs to wheelbarrows to helicopters].
Warning! If you want to avoid frustration open the package before giving and when the kids are in bed or at school, spend some time putting the parts together and taking them apart. When they are brand new they can be very stiff and difficult for kids.
Zoob Jr. for younger kids with softer pieces. I have not tried the Zoob Jr set myself but the idea makes sense.