DS got a big duplo hospital at 3yo, if you are giving the dolls house it may be too similar in terms of small world play but if you choose to wait another year for the doll house then something like that could be worth looking into. He absolutely loved following the instructions (with a bit of help when he first turned 3) to make the hospital up in a number of different ways and since then we've used the pieces to go 'off plan' to make all sorts, farm barn, train stations, airport, hotel etc. Once it's built we use small world bits from other toys such as toy animals for the farm, little character toys for hotel guests etc so it's quite adaptable.
balance bike
mosaic stickers
jigsaws
musical instruments (ocarina, harmonica, recorder - we got DS a zither which he can now play tunes on)
picnic basket set
letter/number/shape stamp set
chunky threading beads
big tub of stickers - DS has animals and like to make 'scenes' on blank coloured paper
cash till for playing shops, post office, cafe
sweetie shop (he was given this, I wasn't all 'that' happy about it being a sweetie shop but we filled the tubs with shiny stones from a garden centre, large marbles, various pasta shapes. I made lots of hand made paper sweetie bags to go with it)
weighing scales (the balance ones with big buckets are fun for water, sand or dry objects such as toys and pasta to be weighed)
second the dress up stuff too