As well as litter tray, litter, food (dry and wet), bowls (but we use saucers more often than not), the things my cats use and love are a cat tree and a cat bed.
Cat doughnut bed from pets at home, the softest they sell.
Cat tree I can’t remember where it was from, but put it next to a window (mine always been by patio doors) so lazy cat can watch the outside.
Later on a microchip catflap, but let them settle in for several weeks before they go out anyway.
Either pet insurance or put away some money every month for pet care.
Flea and worm treatment and annual vaccinations.
Boxes and a few swirly toys and some toy mice.
cheap scratching boards from pets at home placed strategically near stairs.
But start saving up for the new stairs carpet you will need in a few years time.