ditto to most of the above

trains - "toot toot"
cleaning - sweeping and wiping
birds
shoes
water
wearing his hat and noticing other people's hats, including motorbike helmets
clocks
telephones

the main one different to those already mentioned is lawn mowers. He got a plastic talking/singing one at Christmas and ever since then he's been a bit obsessed to the point that he will watch them on the bowling green rather than play in the park

ETA - with the hat thing, he always makes sure to tell other people they are wearing a hat or a helmet