p.s. - If you want to give your cats a taste of the outdoors, the leash idea is a good one. If they don't warm up to that, then simply opening a window (as long as there is a good, sturdy screen that won't give way if the cat pushes/falls against it - or, better yet, windows that open on the top rather than the bottome) and letting the cat feel the breeze and smell the fresh outdoor scents should be stimulating enough.
If you own your own home, you can also build an enclosure for the cats.
If you own your own home, you can also build an enclosure for the cats.