I'm a long-time cat owner, starting in my childhood. I've raised a cat from a newborn kitten and I've rescued adult cats from Shelters hoping to improve the quality of their lives. Through all these experiences I've learned to not underestimate a cat's need for stimulation and a physical outlet for stress.
There are a lot of inexpensive remedies for the bored cat, ranging from a paper grocery sack to a shoe box or cardboard tube. While these can be fun for awhile, the end result is often unsightly and messy. Although real toys are more expensive than simple reused materials, the combination of color, sound, shape and utility really make quality cat toys worth the price.
If you're looking to grow a collection of things to entertain your cat, I have some suggestions. Consider starting with a small cat tree that includes a place to perch, an enclosed area for hiding, a scratching area and (if possible) a hammock or pouch for form-fitting comfort.
Another one of my favorite cat toys is a crinkle tunnel. This type of toy has a cylindrical shape that's flexible so it folds up easily for storage or travel. Crinkle tunnels usually have a couple of types of fabric to add to the play experience and may even have a couple of round cut-outs for added fun during play. My cats love to enter the tunnel from one end and race out one of those holes!
These are my best suggestions for bringing some diversity and fun into the lives of your indoor cats. Consider buying a cat tree and a crinkle tunnel for your cat to play on. You'll be glad you did!