Everything for a cat that isn't food. Litter and trays, scratchers, toys, bowls, cat flaps, carriers and grooming. Litter is the bulk of it, because it's the thing people rebuy, and the rest is mostly bought once and kept for years.
Where to start
Cat litter and litter trays. Trees and scratchers. Toys. Bowls and feeders, cat flaps, carriers and grooming.
Two things that cost nothing and fix most problems
One litter tray per cat, plus one spare. Two cats need three trays. It sounds excessive and it's the single most effective thing you can do about a cat not using the tray, because cats won't reliably share and a soiled tray gets avoided.
Keep the tray well away from the food and water. Cats won't toilet where they eat, and a tray tucked beside the bowls is the commonest reason one goes unused. Different room if you can, opposite corner at minimum.
Neither of those costs you anything, and between them they solve more than any product on this page.
Scratching posts: height and stability
A cat scratches to keep the claws in order and to leave a scent mark, which is why they choose visible places rather than tucked-away ones.
Two things decide whether a post gets used. It has to be tall enough for the cat to stretch out to full length, which is taller than most people expect. And it has to be solid enough not to rock, because a post that wobbles under weight gets abandoned after one go and the sofa gets the work instead.
If a cat is already scratching the furniture, put the new post right next to the spot they've chosen rather than where you'd like it to be. Move it gradually once they're using it.
Water, and the cats that don't drink enough
Plenty of cats drink less than they should, particularly ones fed mostly on dry food.
The simple fix is a second water bowl somewhere away from the food, because many cats won't drink next to where they eat. A drinking fountain is worth trying if that doesn't work, since moving water gets some cats interested when a still bowl doesn't.
Buy one good toy, not a basketful
Most cat toys left on the floor get ignored within a day. Cats hunt in short bursts and lose interest in anything that doesn't move on its own.
So a single wand toy you use with them for five minutes twice a day beats a basket of mice they walk past. Put it away between sessions and it keeps working. Leave it out and it becomes furniture.
What most people buy here
Litter, and then litter again. It's the thing you'll order most, so buying the largest bag you can store is where the saving is.
Beyond that, a scratching post that's tall and heavy, and one decent wand toy. That covers most of what a cat actually needs from this section.