I have had a few cats that had cat flaps, so they could come and go as they pleased. They slept indoors, often on the warmest lap they could find, a lot. However, they also disappeared into the trees and bushes in the neighborhood for hours at a time and often came back grubby and spent ages cleaning themselves. Pretty obviously a sign they had been doing something engaging.

When I visited my friends who had indoor cats, they always seemed either timid or bored, spending hours just sitting looking out windows or sleeping.

I always found it hard to watch a cat staring out at the world, with their only stimulation/interaction happening in the small amount of time each day when people were at home and not busy with something else or sleeping.