How to support a really intelligent cat?

Title sounds weird but I don’t really know how to describe it, I’ve always grown up around cats/have had many as pets but my guy currently is so smart and I want to make sure he doesn’t get depressed/bored because of it. I’ve had him since he was a kitten and I never actively taught him anything but completely on his own he started getting me to play fetch with him daily and also using the human toilet by himself? (He still uses the litter box for number 2 but only pees in human toilets) I work full time and spend a lot of time away from home, luckily I have a separate apartment attached to my parent’s home and they are retired so they come and check on him/feed him throughout the day, but I’m worried about how he’ll cope when I move away because he gets bored very easily/loves human interaction, I’m planning on bringing one of my parents’ cats he gets along with really well with me to mitigate at least some of that. Does anyone have any recommendations for toys or other things he may like on his own? Or any other tricks/things I could teach him just for both of our enjoyment, the fact he just started doing things that a lot of people spend years attempting to train other animals to do was so crazy, I’ve never had a cat interested in potty training or fetch.