I know ‘fun’ is subjective, but I find myself playing city/colony builders or something turn-based with lots of stopping points or pause ability.
Some examples are Kenshi, Civilization, Stellaris, Manor Lords, Baldur’s Gate and Divinity. Mind you, this is typically only while I’m farming. If I’m questing or doing something that requires my attention I just put a show on my other monitor.
Before playing this I was hooked on Hypixel Skyblock for almost a year and I’ve been leader of a guild in Cookie Run: Kingdom for a while now too. Could play either of those in your downtime (though my PC couldn’t handle that) I also played a bunch of Melvor Idle which is heavily inspired by Runescape.
There’s also always more Runescape, if you enjoy the limited (and free) aspect of this you might enjoy F2P only Ironman mode in OSRS. Much more fun in one of the big clans either “f2p im clan” or “w385”.