Monument Island has three major features along the top: the north point of the island is topped by a large oak tree, the center a pickaxe, and the southern tip a functional lighthouse.
The island is surrounded by stone walls to prevent mobs from climbing up it. At the northern and southern tips are small beacons that are always lit. These help define the boundaries of the bay.
The oak tree has glowstone at its base on all four sides for luminance.
The pickaxe was made from oak and spruce planks for the handle, and gray and dark gray wool for the head.
The lighthouse was constructed primarily from sandstone and brick, with redstone lamps and oak half slabs at the top. A clock generator rests beneath the lightouse in its foundation, which is how the lamps turn on and off periodically.
The pickaxe was the earliest feature along the island's top, predating the lightouse's construction in November by about a month. (The northern and southern point beacons were added first, however.) The oak tree was added last to balance out the look of the island.
Due to a bug in either redstone behavior or in the Minecraft server itself, the generator will periodically freeze itself. A button has been added somewhere in the CCA that will send two pulses to the lightouse: the first is supposed to clear the locked up generator. The second restarts the clock. (However, due to the way the freeze sometimes occurs, this method is sometimes unsuccessful, requiring direct intervention in the lightouse itself, using the door on the northside of the lightouse.)
At one point, michaelericbrown theatened to build a large statue of himself before his better judgment caused to him to build a pickaxe instead.