We really don't have that much, we got a brio starter set, station and several trains when he was 2, a tesco tunnel/more track and that's about it. Have bought some smart track and a smart engine, but I was also thinking of a) bridge b) crossing or c) turntable as well as a lot more track from tesco!
It's impossible to know what's good from the huge list available - any help appreciated! I see it as an investment as ds2 (11 mo) already interested...