Claire,
As promised earlier - a few suggestions.
Climbing Frames
Cats really appreciate a three-dimensional environment, as evidenced by their constant attempts to climb up on anything high. Cilimbing frames placed in strategic locations allows your cat to elevate his position with ease and obtain a panoramic view of the outside world. This is akin to having a cat newspaperp as they survey all around in safety and catch up on the latest comings and goings. Cats love watching life.
Bird Feeders
The instinct to watch and stalk birds still courses through cats' veins even though it may have been generations since they relied on catching prey for a living. The provision of window feeders for birds can provide cats with a lot of viewing opportunities at no risk to the birds. Alternatively a bird feeder near your flat window assuming that you are on the ground floor.
Fish Tanks
First and foremost you need to be careful and have the safety of the fish firmly in mind. For similar reasons as the bird feeder, a fish tank (with its lid firmly attached) can be another great pleasure for cats. Even though they never catch the fish, that failure does not detract from the thrill of “fishing.” It drives their curiosity and imagination.
Food puzzles
Traditionally before domestication, cats had to work for their food. Hunting consumed a great deal of their time and energy. We tend to simply put down their food and leave them to scoff in as little as 5 minutes. What do they do then – sleep? If you get creative regarding your cat's feeding opportunities you can spin out those meals and make the process of eating more entertaining. It is diificult in a multi cat household, especially if some cats have special dietary needs. Anyway, Some ideas:
1. Putting your cat's dry nuggets inside an old kitchen roll cardboard tube with the ends lightly taped over and holes drilled in the sides to release kibble intermittently. You need to be careful that no tape is so small that it could be swallowed or become stuck in the throat. Better to be present to see how it works first as a test run. The tube rolls around and is fun to chase and provides exercise in the process which is important as indoor cats don't generally get the same amount of exercise, especilally in a small flat.
2. A plastic cube with various compartments for food that falls out as the cat bats it.
3. Ping-pong balls
Drill a hole in the side to allow you to put a single piece of dry cat food inside.
4. Non-toxic grasses
Some cats respond well to fresh catnip or cat grass grown especially for them. You can buy these in plastic containers from good pet shops. Outside cats can often be seen chewing grass and this helps to compensate for that and help their stomachs if they are a bit off colour.
5. Predatory hunting games for playtime
I am thinking of various feather wands or fishing poles with string attached to entertain your cat. You should probably put aside several minutes a day for this activity. This will exercise and mentally stimulate the cat, and help to re-direct otherwise undirected predatory tendencies and any frustration. A few predatory toys are automatic and allow activation by the cat even in your absence, although I have no personal experience of them. Others on this booard will be better placed to comment on whether they generally work. If don't want to spend much cash there's always the old milk-bottle-seal-on-a-string trick (NB: Be careful to wipe all traces of milk away as ordinary milk is not healthy for cats and they have problems digesting human milk because of the lactose). Alternatively, you copuld leave some table tennis balls around -preferably on a smooth floor.
6. Cardboard Box
Finally, the trusted old box. You could cut out holes in strategic places for puss to spy through or stick his paw through or put some bedding in so it can be a hidey-hole, placed out of the way of noise or the main human traffic, if possible.
I hope this gives you some ideas. Good luck.
