Dibella's Watch adds an entire new continent to Oblivion. It comes complete with a major city, castles, towns, villages, farms, monasteries and all the good stuff that makes up a Medieval landscape. There's also fighting to be had and quests to be made!
Creating interiors used to be my great fear, but nowadays I happily build them all from scratch, placing everything from the furniture and wall-hangings to the bottles and plates on the tables. The most fun part is lighting it up with candles and lamps. One interior like this takes just about an hour to make; then it comes complete with two NPC's too with AI.
This is a picture taken both from the Construction Set as compared to how it looks in-game (the smaller one in the bottom left). It's a new house outside Scarley Village which will house an Alchemist and his wife.