Thomas the Tank Engine Wooden Railway (or other) Bought set for DS at 2.4. was a bit at price but he adores it...has played with it constantly, ignoring other toys bought since...ok he's only 2.7 now so not had it too long but NO other toy has ever had this impact...he's memorised the names of all trains and has learnt to build track himself...great for hand/eye coordination...has been added to loads by us and family in last 3 months...propper thomas stuff from ELC but also Tesco's and Woolies track Will play happily alone (which as a rule he never really liked to do) but dad, uncles and grandpa love it too! Can't recommend enough!
Wooden train track and Duplo/Lego were and are the best without a doubt. Though I have to say, that about that age, DS started on Lego proper and now far prefers that. (He is a little bit unusual among his friends in this, though by no means the only one. Lego, being smaller, takes up less space which is good.)
At just about to turn 3, grown up Lego is the clear winner by a long long way. He still plays with wooden train track and makes up Thomas stories with it. There is more value than I initially thought in having the Thomas and Friends character engines, as opposed to any old engines.
The other things he likes are Playdough, painting stuff of all kinds, Brio Builder System, toy toolkits, Doctor's kit, and a dolls house with Fisher Price people (baby is his fave).
You can get a couple of starter tubs of duplo which have the basics. You can then add other sets etc to it. DS has two basic sets one with windows and one with wheel bases (for vehicles), then this xmas his godmother is adding another set and a larger baseplate. It is played with every day for at least 30-45 minutes, often more. Absolute favourite toy.