My cat basically only sleeps all day and doesn't like to play, what can I do to make him more active?

My cat Simba is 5 years old. He's always been kind of more on the chill side, and mostly just likes sleeping and sometimes cuddling. Lately tho I feel like he's sleeping a little bit too much, or have just started playing less and less overall, and I don't really know what to do. My mom and I have tried doing some enrichment activities with him, like onece we built a little hideout/hut (sorry I don't know what it is called in english), and one time we filled some things with water and tried to make him interested. But everything we've done he's not interested in. He just sniffs a little and walkes away. Same when we try to play with him. We've tried toys, running after him (which some times works), hiding stuff under the mat. Some of it works sometimes, but that's usually when he's already feeling a little playful. But even when we've started playing with him he only does it for a few minutes then walked away and doesn't care anymore. We have also tried to get him a friend, but both of those times he just hasn't seemed interested in interacting with them, and honestly since they have been gone he has seemed much happier. (The first one died and the other one we had to give away). I can't really think of what more to do to get him up and running, and at this point I am started to think he is depressed or something. Any ideas? :[

Oh and also we live in an apartment and have a balcony that's open mostly all the time during the day, where he is pretty often. He only sits and looks at the cars and birds tho, no running or playing (not that there is much space to run anyway).