Cats want to be engaged. Especially those cats who life inside a house and therefore won't see much of the wonderful and ever-changing outdoors. Cats who live in the wild play with natural objects like flying leaves and captured rodents which you can observe especially on kittens.
Of course you can also engage your cat in a way that makes her dull indoor life a little more interesting by getting toys and showing what to do with them. A suitable toy for a cat doesn't have to be synthetic, stylish and therefore often expensive. Wool, old socks and other interesting materials can perfectly be used to build a fun toy for your cat.
Cats want to play!
A soft string (best suited are, of course, natural materials like wool) as well as hand-made soft objects that looks a little like a mouse for example are great toys for your cat that often don't cost a thing.
As soon as you have shown your cat the new (handmade) toy your cat will understand what it is made for and use it herself. That will not only be fun for your cat. :)
One thing though: Cats think autonomous. So she might be pleased that you as her servent have brought her a new toy but you certainly can't force her to play with it!
Picture: CC 2.0 by blackplastic