The pot for tyhe contrast is not strictly needed, you can use a fixed resistor, but the pot is really handy to figure out what size the fixed resistor should be 
Optocouplers and tyransistors are two ays to switch / drive stuff. You might need other things depending on how much current the things you want to control draw. Power transistors like TIP 120, or relays for instance. But i really depends on what you want to control.