Magnatiles. We have bought many sets over the years and they have grown with our boys, now 6 and 8. I think they like them so much because they can experiment with complex geometric shapes by building from the basic triangles and squares. They can be built large enough to house Star Wars ships during imaginative play. I've even seen them build a maze system for a little lizzard they caught outside.
It's such a simple, basic building toy but the geometry and size seem to keep them endlessly occupied. They are a little younger- however it's the only building toy they both play with every week for 4+ years. The structures are also just delicate enough to hone fine motor skills but sturdy enough to avoid major collaspes (though they happen.)
Check pinterest for inspiration. Duck tape, paint, and large appliance boxes you can get for free from the local appliance store have remarkable possibilities!
