Go to 1429 74 3894. That's right; it's a mushroom biome. Then dig straight down until you reach the dungeon, next to the ravine. Take the items from the chests, if you want, and then build a Nether portal in the dungeon. You will find a Nether fortress on the other side. If you have time, go back and mine your way to 1386 28 4049, to find the other dungeon by the abandoned mineshaft.

So, a mushroom biome, two dungeons, a Nether fortress, a ravine and an abandoned mineshaft, all in ONE island!