
I know just how addictive it is

Hmmm... For a follow-up, it's difficult to recommend something. It depends on what you'd like, and what skills you'd like to improve.
If you want to work a bit on the bling factor, Elmer's Fancy is a great build.
If you want to test your skills at making something small, Tiny is a great little engine.
For a really nice, but longer-term project, I can really recommend the Grasshopper.
Then there's also always the option of making a bit of tooling you need

My best advise would be to follow your heart and build something that draws your interest and that you really want to build

, Arnold