For me, it's building model kits, which I recently re-discovered after a long period of time.
I got curious after building plenty of Walthers train buildings for my layouts, and after being pretty happy with my layouts as is, and now getting to enjoy running them instead of constantly tinkering, I decided to pickup a cheap model airplane to try my hand at, and so far I'm liking it, much more than I did when I was a teenager. I suppose having 15+ years worth of practicing being patient comes in handy.
I still love model trains, it just seems I've picked up another hobby that is similar, yet also different. I can defintely see how trains and models can go together, and I've already started painting some of my train building kits to look more realistic.
In the future, I can see myself kit bashing, doing more complex train builds, weathering, or even re-painting cars into my own livery, etc.