I have two cats ( I think for indoor cats in apartments one is going to be lonely) my cats have their own room, in which my boyfriend has put together a run, where they can climb, chase eachother through tunnels, with scratching posts and all their toys, the rest of the apartment is usually littered with their toys aswell, where they've abandoned them in favour of something more interesting, like a bottle top (LOL - isn't that always the way, get them some cool looking toys and they'll always end up playing with the wrapping!!!)
My apartment has a pretty long hallway which is great for throwing the ball and for them to fetch which they both love, especially Misty, she loves rubbery really bouncy balls!!
And then they also have eachother and they do love eachother alot, they play-fight and chase eachother at all hours and playtime is always a highlight for me because they are hilarious!
I think as long as you prepare for it, with enough things to keep their attention and happiness, and as long as they get loads of love, they have very meaningful lives. My two are at this moment working on a new psychological invention - how to hypnotise humans into doing whatever you want - so you see they spend their time in very worthwhile ways! (LOL)