Minecraft wiki entry for Time-saving_tips, associated text: Fill an area with still water, then pour lava over it. This allows you to quickly pave a large area with smooth stone without having to collect great volumes of cobblestone, smelt it back to smooth stone (requiring fuel and furnace time) and then place it. A variation of this is the lily pad trick for making a quick island in the sea.
Mining obsidian, even with an enchanted pickaxe, is slow and tedious. If you want to build something out of obsidian, it is far more efficient to collect buckets of lava, make a mold of some cheap material, fill with lava, and convert to obsidian en-masse by pouring water on top.
To tower down from a high place, pour a bucket of lava down, then a bucket of water beside it, then collect both water and lava back into their buckets. The flowing lava will have turned into cobblestone, which you can stand on and mine downwards.