Random Traps for any intruder you don't want in your Minecraft world/mobs?
Automatic Piston doors in your base, but not hidden, no real use, Just for show.
Also, you can craft dispensers if your in the latest snapshot to pour out water to make Semi-Auto/Automatic farms, Without the pistons.
A creepy room, Filled with mossy-cobblestone, Lava, and random bits of redstone here and there to make it look like blood? (Kinda depends on your texture pack thou.)
Have the on a rail achievement? You can use Redstone to make powered tracks to get around your world easier. Although, it does require a lot of Gold, and/or Iron if your thinking about using detector rails instead of redstone torches to make it look better.
And thats all I can think of.