I’ve written before about how to offer the indoor cat the wonders of the out of doors. You can leash train your cat and take her for walks in the neighborhood or into the wilderness when traveling, for example. You can build a catio—a screened in room with a grass floor and some of your cat’s favorite plants inside. Or you can fence your yard and string an electric wire across the top of it to keep the cats inside. In any of these cases, be aware that you’re exposing your cat to fleas and ticks (in certain areas) and diseases that any free-range cats might bring around.
Most who keep their cats indoors simply make sure 
their cat has a wide view of the out-of-doors and fresh air coming in for them to feel and breathe. Here, we have a window perch for our cats where they can sit and look out the window comfortably. We open the windows when the weather allows, making sure the screens are secure. And we provide birdbaths, bird feeders, plants that attract birds, etc. to entertain the cats (and us). We also keep curtains and blinds open during daylight hours because I like a well-lighted room and so do the cats. If you have a particularly curious and maybe gregarious cat, you might look into cat-proof screens or grates to discourage cats from escaping through an open window.
Here are a couple of sites that help you to leash train your cat.
https://happycatshaven.org/knowledgebase/how-to-leash-train-your-cat/
https://www.adventurecats.org/backcountry-basics/train-your-cat-to-walk-on-a-leash/
