The most important thing to remember with cats and their toys is to offer them a variety.
Of course the cat is bound to have a favourite toy, they all do, however you can keep them very happy by replacing those toys that are worn and old with new ones.
Many of the new toys are associated with catnip in some way. Over time the catnip will get old and essentially be less attractive to your cat, as far as actually playing with it goes.
Another toy they will simply love is scratching posts. There is a web site "superstore" for those individuals that reside in the UK that offers several different toys for keeping your cat happy. Simply do a search for superstores and pets in the UK and the results will include the web site.
These toys should also stay clean, although they are known to clean their toys themselves. They can quickly get frazzled and have strings hanging off them that shouldn't be there. These should actually be removed, or as stated above, if the cat toy is too worn or old just replace it with a new one. They love to play with toys that have bells on them or certain sounds. It makes them extremely curious and therefore playful.
Keeping your cat happy with various toys is also the key to a healthy pet. You will find they play often and stay fairly active using these toys which is a very good form of exercise for them.
You will find kittens are always getting into something like bags and other items such as boxes. Now there are toys that are made that are somewhat like large columns with holes in them. These types of toys are better for your cat, as bags and boxes can be become a danger to them. Safety is another way of keeping them happy with their toys.
For the most part many of the toys, scratching posts and columns as well as the vast variety of others, are reasonably priced. Many of these can be replaced on a regular basis without getting too expensive. However, your cat will love you because of the new catnip smell they will obviously be aware of.
Keeping them happy with toys will also keep them out of trouble. For instance, some cats get bored and start to claw furniture if they don't have scratching posts, or they will climb up on curtains; this is where the toy columns would be a good idea.
The toys should also be the type that a cat can paw at and sometimes even bite. Believe it or not, some like to carry their toys around with them. So if the toy is entirely too big, don't be surprised if they don't play with it very often. The key to keeping them happy is to ensure there are plenty of toys available.