The top toy in our household has been small plastic animals. DD got them when she was 18 months old, and still plays with them at 4. They're great for imaginative play, and can go in the bath/swimming pool. We've taken them on airplanes, trains, cars etc. They go with us everywhere. We have farm animals, jungle animials, sea animals, dinosaurs, etc.
Don't know if that's the sort of thing your ds would enjoy, but it's been (and remains) her favourite.