Well done for considering what's best for your potential cat w This might get a bit long but I'm studying to be a feline behaviourist and have 2 indoor cats so hopefully you'll find this helpful
In the US, Canada, NZ, Australia, etc allowing cats to freeroam has been losing popularity for years due to the risks posed to the cats and the havoc they wreak upon natural ecosystems domestic animals have no place in - even being made illegal in some places, but the UK is very slow to update in regards to pet care standards (despite over six hundred cats a day being hit on our roads).
Cats require mental stimulation and physical activity like any other pet, but due to the cultural norms surrounding their roaming many owners fail to realise how much enrichment they require - leading to situations where they cat becomes depressed, aggressive or destructive and the owner blames it on them not being outside, rather than the true cause: boredom.
Enriching their environment - consider how to make your living space as "cat friendly" as can be. Do you have room for a cat tree or two? Putting up shelves they can climb and perch on is fantastic also - provide them with vertical space. A cat that can survey their territory from up high is a cat that is secure in their territory. Things like cat grass, toys that don't require human input, rearranging things to provide a route to the top of high furniture, etc.
If you can, cat proof garden fencing or catios can be DIY-ed to provide safe experiences outside - Amazon even sells cat tents for this purpose.
Things like puzzle feeders and lick mats can provide entertainment for a while as they work at getting their food, and these are sold for both dogs and cats (Cat specific products are usually more expensive for some reason, but things for small dogs often work just the same). There are some forms of puzzle feeder that can be made at home, or even games with hiding some dry food. We have a collection of cardboard boxes for the cats and often scatter some dry food in a few different boxes. They hear it and come running then spend time jumping from box to box to find it. Training also fits into the mental enrichment category. Cats can be taught tricks and this is both mental exercise and a good bonding activity (Plus people love a cat that knows how to high five)
Play should be done daily - if you can, just before (scheduled) meals to follow their natural rhythm. The aim is to replicate the act of stalking and chasing prey, both to give them an outlet for their natural instincts and use up some of that energy. Cats sleep so much because when they go they really go. Intense playtime is how you bring down some of that excess energy and prevent the kind of mischief a bored cat will inevitably find.
Hopefully this gave you some ideas :)