Boxes, my kids loved boxes! Often more than they toy that came in them. Plus brown boxes can be anything, encourage them draw on them, make fairy castles, forts, houses, spaceships. We took countless trips to the moon and planets with my kids in a box. Add in a few blankets and the possibilities are endless!
Adding to this, go for simple toys, wooden blocks are great. Legos are awesome for slightly older kids. Encourage them to be creative, one of my sons friends would buy Lego kits, build them, and put them on self. I say build it, break it, combine it, create it.
I haven't bought them many toys; I don't want to buy them tramline toys. The local toyshop is Hamley's; I couldn't see anything in that store that wasn't both a tramline toy, and entirely made of non-recyclable plastic. I had to import the wooden blocks.
When I was a kid, I got a lot of mileage out of Meccano. Even into early adulthood, I was still using Meccano to botch together some project or other. I suppose Lego is the modern alternative; but (a) it's all plastic; and (b) modern Lego is all tramlines kits.