Building a glass island in the sky above my castle. I'm actually finished for the most part with both of them, though I need to figure out how to get animals into the livery and finish the armory (including stocking up several chests with weaponry). I've actually got the beginnings of a nice stockpile under the stable. I've also constructed more than half of a sky bridge between my mansion and my castle. I've died so many times, completing it. O_O
I must say, I rather enjoy having a huge castle to return to at night. Far, far better than the little mining shack I had earlier.
I've also just about decided that following natural caves and mining from and within them is much more likely to yield resources than just picking a spot and digging. An hour in my self dug mine yielded about 20 pieces of iron, 40 redstone, 6 gold and almost enough cobblestone to construct a small castle. Mining from a cave netted me around 60 iron, just as much redstone, 10 gold and 2 diamonds in about half the time. Maybe everyone else already knew this, but...I did not. XD