I don't have an Olaf, i have a Felix 
But something along the lines of Ikea wooden trainset (still played with by my 7yo and in excellent condition). It can be added to with Brio, Lidl, Wilko, all sorts of other brands.
Really anythung Ikea is fun - dress up, wooden kitchen, fabric food, tool kit, colouring and craft stuff.
DS received a multistorey car garage by Molto at that age, i think. Fabulous thing- had a wind-up lift, petrol station, car wash etc. Played with it for years, it's just been regifted.
Playmobil is nice from this age, nothing with too many pieces like the campervan, but car with boat or mini zoo.
Schleich (sp?) animals/dinosaurs/Smurfs are amazing quality and can, over the years, be added to in Christmas stocking or from museum shops etc.
Orchard Games make good quality and simple boardgames for this age, we have Pugs in Pants still a great hit!