I use a couple of little shopping baskets (like kid's play baskets but these were adult version intended for cleaning products I think, they are sturdier than many kid's ones but still small and cute), and a stack of small baskets (or tubs) so i can put a pile of stuff in them when needed. A light weight tool box is great for keeping all sorts of art materials as they have different compartments but the whole thing is packed away easily, art students tend to use them too.
I have a set of A4 paper trays, stackable, like in-trays, great for keeping different sorts of paper and blank cards etc on each shelf, and a set of stackable plastic crate type things. Oh and lot sof those plastic tubs with lids like you get take away food in (£ shop or wilkos sell them too) which stack well and hold all sorts of pens, stickers etc.
stencils, shapes and letters
stampers, shapes and letters
oil pastels
various types of paint
various bits of stationary like little sets of highlighter pens, bingo dabbers (£ shop) and all sorts of cheap little cute stuff, we have pens which are felt pen at one end but a roller stamper the other end, they don't last super long but DS likes the 'difference' of all these things to experiment with. Also various little stampers which are again just cheap but again add to the wide range of 'stuff'.
I use adult art brushes, not expensive ones, but even so the quality is better than many kids brushes. You can usually get a set of hog hair brushes a the £ shop, and cut one or two down to make them more stubby for stippling through stencils etc.
I get blank cards with envelops from The Works, we always make cards for friends/family birthdays xmas etc.
A roll of backing paper (wall paper backing) is great for larger work, or a roll of brown paper.
Tissue
coloured paper
card
A2 heavy weight cartridge paper or water colour paper
coloured pastel paper is much nicer than sugar paper, although pricier.
Things to stick like mutli coloured feathers, buttons, lolly sticks, multipack of coloured drinking straws (100 from £ shop) - good for scissor practice too.
Hole punchers and shaped hole punchers are nice and you can stick all the little bits onto another paper.
Googly eyes - DS loves googly eyes!
TBH we have a ton of stuff and DS isn't all that interested, I have to set up a special project together for him to do anything but he can get very involved if I catch him at the right time with the right thing. Variety is key here!