I went on a small server and helped some guys build a small town, went to work for two weeks and it was a gigantic metropolis when I got back on. I have yet to find any other interesting servers, they're either too big to make any kind of impact at all, or pvp.
Singleplayer is pretty uninteresting, though it gives a huge timewaster. I flattened a large island and built a small dock to travel to the next forest island. Excavated the hell out of the forest island and set up 5 furnaces churning out charcoal and smelting cobblestone back to stone blocks. I then built a massive castle with large symmetric towers on the flattened island.
Quickly got bored once it was finished (took me a week of daily gameplay) and started a new world. Dug straight down until I hit bedrock then started mining around randomly until I found enough diamonds to make a pickaxe and a sword, packed all my stuff and headed out in a random direction in search of lava. After a few game days of walking I came across a hollow half-mountain with a lavafall inside, pretty sweet view. Dug out a 7x1 moat and made a bucket to fill it with lava. Some water on it and boom, obsidian. Used the diamond pickaxe to mine it up and repeated the process, until I had enough obsidian to build a portal.
So now, I'm in the nether. Alone and bored, just like on multiplayer except with more fire.