On the Koala Wallop multiplayer server, I've been hollowing out a big room deep underground using TNT. A lot of TNT. Luckily you can exploit server lag to duplicate items, which is what I've been doing. For this trick you need a large chest and any block type you can afford to lose, like dirt or cobblestone. Make two stacks of blocks, place them next to each other, and put the item you want to duplicate under it. Then quickly merge the two stacks, and then quickly double click the other item. The merged stack will now have half as many blocks in it, and the other item's count will have doubled. However, it will only work with duplications of 32 or less, since stacks of more than 64 are not acceptable.
Anyway, in this room I'm building a pyramid made entirely of lava. A pyromid! For this, I first made a template pyramid of cloth on a base of cobblestone. As soon as I'm done making the inner chambers, I will put a lava source on the top block, which will then pour over the entire pyramid. Afterwards, the cloth will burn away, and the inner chambers will also be covered in lava, which makes for a convenient light source by the windows inside.
The inner chambers will be made of obsidian and glass. I'll put some treasure in there, some decorations, as well as little statues of men and of course, a sarcophagus. I'd be working on it right now if only I could log in.