My youngest is less of a toy person than my oldest. I mean, he plays all day long, but he mostly uses things as props in a pretend scenario.
My oldest was always very mechanically inclined. So, for him, we have two big tubs of Duplo, one tub of regular Lego, a tub of K'Nex, various Lego kits, some Mechano, big wooden blocks, etc. My little one will use the Duplo and Lego sometimes. Duplo (the chunky Legos with 4 - 12 pegs) are so open-ended. We prefer the tub over the kits. Get something with wheels. My mother bought the Thomas and Friends train kit for Duplo but neither of my kids took to it. It only had a few pieces and you can't do anything with it but make the same train.
Both my kids love pattern blocks. We have a tub of painted, wooden pattern blocks: triangles, hexagons, trapezoids, parallelograms, and squares. Both my kids loved these from age 2 and up. You can get puzzle sheets, but they are probably more fun to use in your own way. Without the puzzle sheets, kids can still make designs or specific pictures. They can stack them up. They are wonderful. I think we bought ours at ETA Cuisinaire, but there are various types. I think Rainbow Resources sells them too.
We don't seem to get these out as much as we could, but we have connecting math rods. You can make "trains" out of those too.
We have some Waldorf playstands and they are extremely popular. They are a bit expensive and they do take up space. If I had it to do over, I'd only buy one instead of a whole set. They are great for kids who like to do a lot of fantasy play. My kids have used theirs as a house, a restaurant, a bus, a museum, a toy store, a spaceship...You can get some big playsilks or sheets of fabric to drape over them to make a roof and doors.
Puzzles are very popular in our house. We have tons. I will only buy Ravensburger now, for quality reasons. Also, I discovered that we loathe floor puzzles because the child has to crawl across the puzzle to solve it and that disturbs the pieces. There are some gorgeous Thomas the Tank Engine puzzles out there. But beware of buying too many in one piece count. Also, buy a piece count that is slightly harder than you think your child can handle because it lasts longer and there's less of a boredom issue. Also, check the complexity of the picture over the piece count, because some high level piece counts have easier pictures than lower ones. We had a set of 12 piece Melissa and Doug jigsaws that were harder (per piece) than some 35 piece puzzles we have.
Art supplies are always good...paint, clay, good thick paper, watercolors, popsicle sticks, etc.
HTH!