He has discovered the joy of trains and tracks. We were at my sister-in-law's house for a last meal together before we left to come home, and she pulled out their long-outgrown tracks and trains. He spent ages driving the train around the track we built ("chugga-chugga-choo-choo"), and then tried assembling his own track.
I guess it's time to dig out our trains...I just have to figure out what to put away to make room, first.