Indoor cats generally live a longer and healthier life than outdoor cats because they avoid many of the dangers associated with the outdoors (e.g., injury from traffic and other animals, ingesting things that are toxic, catching diseases from other animals, etc.). The wildlife is often safer as well when cats are kept indoors because of a cat's hunting instinct.
However, your indoor cat is also missing the stimulation that comes from being outside.
This article mentions a number of ways that you can give your indoor cat an outdoor experience.
Walking your pet isn't just for dogs anymore. With patience you can teach your cat to walk on leash too.
Make sure your cat is healthy enough before you venture on this route. Also, not all cats will enjoy this activity.
Sling carriers and backpack carriers can be used to carry your cat on walks and hikes. Pet strollers allow you to push your cat along with you on walks and even jogs.
These can be good options if your cat has difficulty walking for health reasons or otherwise. They can also be combined with walking your cat on leash.
You can use an ordinary bike basket or one specifically made for pets to take your cat along with you on a bike ride. You can also tow your cat on bike with a bike trailer made for pets.
Some baskets and most trailers have a quick release mechanism so they can be disconnected from the bike allowing you to use your bike normally when not taking your cat along with you.
There are outdoor enclosures that you can set up for your cat connected to a door or window. A standalone enclosure can be set up in the backyard or on a porch or balcony. You could also build a more elaborate catio (cat patio) or piece together a system of tunnels and tents outside.
Create comfortable window ledges for your cat to sit on and lookout. Add feeders to draw birds and other animals outside the windows to give your cat something to watch.
Give your cat a sort of simulated window experience by playing videos or live streams of animals, birds, and insects outdoors or fish underwater or in an aquarium.
How do I get my cat used to wearing a cat harness?