I'm having a lot of fun. Exploring caves, building mine cart tracks (yay for the physics "bug" that allows boosters!), finding diamond, but my favorite part has been constructing:
My Rooftop Garden.
I'm not sure why I like it so much. It's very peaceful up there. It feels a little like decorating my house in Animal Crossing.
Here's what my tower looks like:
The image was made with Cartograph, which is a great tool (but also a temptation to "cheat" when mining ore.)