Is there something like an ignition key for planes?
Or could one "just enter" and fly away if they could?
The planes I have trained in (single engined Piper Cherokees/Warriors) do indeed have an ignition key, without which the magneto system (which drives the spark plugs) and the starter cannot be engaged. Without being certain, I would assume the same goes for other small GA aircraft.
When you get to airliners I have no idea whether there are keys for locking the controls - as the controls are much more complex, the keys probably won't be directly connected to ignition systems or starters, and I wouldn't be surprised if the primary security method is keeping doors locked.