Over the past two years we've unwittingly accumulated several sets of wooden blocks. They're great, but they're uniform in size and shape and our son has moved past the stage of simply stacking and knocking them over. He's ready to build something! So we have our eye on this super cool castle block set.
He might need more than one set to build a full castle, but this is a great start and very affordable at $22 for 9 pieces (plus a carrying bag). You can find them in the Etsy shop Pin Pon which has a lot of cute wooden toys (like this smiling bowling set).