For me, it’s just maintaining volume of riding which Zwift helps with.
I started by doing the challenges (I did nothing but climbing hills for 4 months to get the Z1), and working through all the routes. I have only the six hardest ones left and I think I’ll leave those for winter, or group rides for company and drafting.
Achievement badge hunting is another that’s been fun. I posted about my mission to get the 100kph badge and it sparked a great discussion on different methods for getting that.
Mixing up different routes helps with variety - pan-flat are good for steady-state but rollers like The Esses and Titan’s Grove are more intense.
There are some great organized rides by SZR (Swedish Zwift Riders) and Herd. The latter has a “Bull Run” that helps bridge the gap between Cat D and Cat C - as a solid Cat D rider trying to move to C, Bull Run was intense and awesome. GREAT energy and communication from ride organizers. THAT stuff keeps me interested. (of note: I don’t race at all)
There’s a pinned resource guide with some useful and fun tools here: https://forums.zwift.com/t/links-to-zwift-resources/