Ideas with Redstone

A gun that permanently shoots fireballs when you flip the switch.

A cellar that can only be accessed by a hidden lever.

A fireplace with a redstone starter in a compact design.

A trap that burns unwanted visitors.

The trap is activated as soon as someone removes a block (here diamond).
The picture shows a cross section so you can see the inner workings. Normally, the floor around the diamond block is naturally flat.
The trap is based on the fact that you can set levers on block bottoms.

You throw your garbage in the trash can and later the garbage collector comes and picks it up.

Realistic short distance transport of items - allows different uses depending on the design (e.g. assembly line, conveyor belt or drainage channel).

A simply constructed machine that makes teleportation home or to a specific location the easiest method of travel or escape!

A very compact secret door in the floor.

If the player approaches the diamond block, maybe takes it down, he can't get away because a command prevents him from doing so.