At that age mine loved playing with a play mat - it had roads, fields, ponds etc painted on it (I made it actually but you can buy them) and I bought a selection of plastic farm/wild animals, little cars, dinosaurs, and people (police officer, firefighter, lollipop person etc) and they used to make up all sorts of games using the mat as a base. They also liked the sandpit - I had one that had a lid that was like a terrain with hills and craters etc on it - they used all the animals cars etc on that too.
They also loved dressing up (I bought stuff from charity shops - bags and hats v popular, an old phone, a purse etc) but you can buy ready made costumes if you prefer. A doctors kit is good for pretend play, (they loved wandering around with a stethoscope round their neck diagnosing their teddies with 'common diarrhoea'😃)
Playmobile and Sylvanians were popular. Jigsaws, books, dolls/bears and accoutrements (cot, bottles, pram, old baby clothes to dress them in etc). Art and craft supplies.
For outdoors - gardening bits and pieces, spade, bucket plastic wheelbarrow. Different sized plastic balls for kicking or throwing. Plastic golf set with big clubs and a ball to whack ! I've seen mud kitchens in shops - mine didn't have one but they look fun!