Well, there are practical and impractical things. The practical ones are a lot easier than the others, and you may know how to do everything that would be useful for a survival character already... automated farms, and whatnot.
Some of the more interesting devices would be, say, an automatic ice generator, or different chunk-reload and game-reload-proof clocks. One-clocks being probably the most interesting of those. I'd suspect you know how to do those, though.
Making a self-regenerating cobblestone arena or whatever-else is more of a medium-level task, and is ... somewhat practical. Um... you could try a door with a combination lock - three buttons having to be pressed in a certain order to open a door. That would be tricky, too.
If you can come up with a design for a smoke machine, though, that would be pretty amazing.