We do a mix of more structured activities too (not all of the time, but the odd half hour here and there).
My same age ds likes the Shopping list game (orchard toys) and I've just bought a couple of others: Match and Spell and a Westher game.
We play rockin robot (Elc)
Build octogons, play with threading buttons, and have one of those cork boards that you tap shapes and tacks into.
His big brother is brilliant with him too (he's 11) and they'd a lot of Lego and we have these giant blocks from Melissa and Doug (they're cardboard but so sturdy) and they become rockets, forts, dens, race tracks, you name it.
And the other toy that he adores is quercetti tubations. A huge hit.
Otherwise he runs riot, playing in the mud, building rivers with the hose pipe, and playing with cars, trains and animals. And the dreaded iPad!