Some cats enjoy 'cat tv'. Literally. There are dvds available that focus on birds, or fish, or mice, and some cats find this very interesting. Kato didn't even look once at such a video, but I have a feeling Ava will be different, as she actually watches the activity happening on a computer or tv screen. Then there are the 'real' versions of cat tv...fish tanks and other animal environments behind glass. My feline specialist vet has a fish tank in her consulting rooms just for the cats! If you go this route, you may need a cat with a lower prey drive, as I have a feeling clever, willful cats like ours, would work out how to get the prey.
What is unfailingly stimulating are windows to the outside world. For a year, we had our bedroom window shut behind homemade sheet-drapes ( I like a dark bedroom). But I finally got myself in gear and transformed the sheets into drapes that could be moved...and wella, an amazing head-height window and window-sill appeared, which, as a bonus, is situated on the second floor and looks out over the roofs and backyards of the neighbourhood. That translates to BIRDS! And sometimes cats. And people and workmen. Some people get special window-sill attachments so their cats have a plush perch or bed on the side of the sill; we can't risk the possibility of dents/scrapes/screws as we rent, but fortunately, the sill as is still provides a good sitting spot.
If you don't have window sill for a good observation outpost, consider moving your cat tree close to a window.
Kato loves our upstairs window ledge and waits every morning for me to get out of bed and open the blind and drapes. The morning sun also hits that spot so it is lovely and warm. If I hear chattering up there, it's birds. I have had an odd occasion when not-happy sounds were made, followed by Kato jumping down and running downstairs to the door to the courtyard, then zooming back upstairs again. This was all accompanied by unhappy muttering and odd yowls. When I lifted myself up to the window, I saw what the excitment was...Kato's arch enemy Ginger was on the fence below. The same fence that encloses our downstairs courtyard! (Seriously, I don't know why they decided they don't like each other; Kato will peacefully watch the other neighbourhood cats.) One caution about this. Some cats will start to mark (PEE) their house to proclaim their territory over outside cats, so it may be wise to make sure the neighbourhood cats can't encroach on your territory. For us, the courtyard is enclosed with chicken wire, so cats can't enter, and Kato can't escape.
And that leads us to outdoor spaces. Enclosed garden, balcony or courtyard places are wonderful for cats. Today we sat out on the balcony and both little cat faces lifted into the air with slitted eyes and sniffing noses. Zephyrs of scent drift by. And birds fly overhead, which invariably sees the cats jolt down into a watchful crouch. The Indian Minor birds actually have a patrol roster for Kato and send up a squawking alarm, followed by croaking. These birds are the other arch enemy, and Kato chatters back at them. Ava is yet to learn about these 'special' birds!!
As mentioned, we also have 'meshed in' our courtyard. You can get professional cat-mesh jobs like these guys (and many more - just google), but as said, we rent. So we had to come up with an option that did no damage and could be taken down when we leave. I've written more about this in my post about 'Cat Enclosures' in the Equipment & Furniture section. The end result is a garden space that the cats love to lounge in, pounce in, and go to get cool or soak up the heat and humidity. They also like to catch flies and moths...and dream of birds being silly enough to fly down through the mesh!