Lego Ninjago
My 4-year-old son has already been playing with Ninjago at his after-school care. Now he also has some of this at home. He absolutely loves it, and even though he's too young to fully understand the game, he creates elaborate stories with his Ninjago toys. He also plays with it when his school friends come over, often combined with other LEGO sets or even with Thomas the Train (the LEGO figures fit perfectly in a Thomas the Train wagon). It's a big hit at our home.