Written for Malachain: the essentials, without the noise.
Cats in flats need height, hiding places and hunting more than floor space. A tall scratching post by a window, a shelf or two to climb, a box to hide in and a daily session with a wand toy cover most of a cat's needs. Feeding part of the daily food from a puzzle feeder turns mealtime into the hunt that a bored cat is missing.
Keep the litter tray far from the food, scoop it daily and provide one tray per cat plus one spare. Secure windows and balconies with mesh so fresh air is safe. A second cat is not always the answer to loneliness; many cats prefer being the only one. Ten minutes of play twice a day does more for behaviour than any toy left on the floor.
There is more to learn on this subject, and the linked website goes into greater depth. Treat this article as an introduction rather than the final word, and check any important decision against up-to-date, reliable sources before relying on it.
If you try any of these ideas, start small and adjust them to your own situation. What works for one person will not suit everyone, and it is fine to keep only the parts that help. Writing down what you changed, and what happened, makes it much easier to see after a few weeks whether the effort was worthwhile.
The most useful step is usually the first one. Pick a single suggestion, try it for a couple of weeks, and then decide whether it is worth keeping. Small, steady improvements last longer than ambitious plans that are hard to maintain once everyday life gets busy again.