It'd be cool to have options like tables, chairs, lamps, desks, and the like, to make things feel a bit more homey.
Tables/ottomans and chairs/couches are pretty easy to fake. Slabs work well for low tables (with full blocks, typically logs, working for higher tables), and a stair block (or several side-by-side) with a blank sign on each end make pretty convincing chairs. Paintings can help liven a room up too. Fireplaces can add some atmosphere to an area as well, though they've required using netherrack as a fuel supply for a while now, not to mention a healthy dose of caution, if you're building with wood. If you pop onto the multiplayer server and stop by my underwater domes and/or Heward's Handy Hovel, you'll see the principle put into practice.
Incidentally, thank you for not neglecting the interior. There's too many cool structures on the multiplayer server that feel like they're going to waste, because once you're inside, there's nothing to see.
Speaking of multiplayer projects (and clever segues), I've built a test-pool in the middle of what will become the Submarena. I've got to re-watch those underwater road videos, because I can only get a boat to stay submerged briefly. I've a feeling it's just going to involve rearranging the slabs on the bottom, but we shall see.
