Use command blocks in such a way that causes someone to teleport to the nether when they walk through a door.

Build an automatic xp farm that pushes mobs off with an on/off switch Mwahahahah

There are millions of things you could do ! Build some secret trap door with piston or a mob/player trap. Create a whole building or castle filled with little widgets all powered by redstone stuff like that! If ou rly wanted to just look on YouTube at channels like sethbling and try and build one of his creations!
Arctic

you could build a tower with traps and command blocks hidden inside