Cheap and Easy Ways to Stimulate their senses

So we all know that keeping kitty entertained and active is important for her health, mind and happiness, right?
After spending countless dollars on toys and items that promised to be the cat's meow, no pun intended, here's the very simple, and very inexpensive, list of what I've found works to keep them stimulated when I am home and when I'm spending my days selfishly away from them at work:

1. Twist Ties. These things are amazing when it comes to playing fetch, or watching them bat it around. Cody knows the drawer that we keep them in and runs over with what we call his "squeak" meow (the same squeak he lets out with he sees a bird) at the sound of peeling one of them off the batch. These are great for self-entertainment, or tie a bunch of ends together and flick it against the floor to play with kitty. We rely on these so much that when I conduct my yearly take-the-stove-out-from-the-wall-to-clean-the-nastiness-behind-it day, I can be guaranteed to find at least 50 ties in amongst the fuzz.

2. In a long, somewhat shallow rubbermaid tote, plant cat grass, rye grass, lettuce and catnip, and place on the floor. In my house, the cat grass goes first and in fact, my kitties wont even let it grow before it's gone. These are all greens that kitties like and help with their digestive system. This also emulates being outdoors, which stimulates kitty's senses. As an added bonus, my cats now leave the rest of my house plants alone.

3. Make re-usable catnip bags (aka "slobber bags"). I was buying these regularly at $2-3 each, just to have them soaking in slobber an hour later and not touched again for a week. Then, feeling kind of disgusted that after sitting there for a week, collecting the fur balls from the corners of the room and crusty with dried saliva, my kitties were again chewing on them. So, if you have a sewing machine, grab a pair of old, thin clothing (say a pair of pajama pants that are starting to fall apart), sew up a little bag, and sew Velcro into the opening as the fastener. That way, you can empty the catnip, wash the bag and use it over and over again. If you knit or crochet, this works too, just make sure there's not too long of a string hanging from it the bag, so that you can be sure they wont swallow it. Nothing gets my kitties moving (or, uh, rolling on the floor) like these bags.

4. Put a bird feeder near a window where the kitties normally perch. I like to think of this as television for them. Or torture, whatever works.

Any other suggestions and tricks, and I would LOVE to hear them!