a bottle of white vinagar and water.
4 caps of white vinagar in a bowl and turn it up to full blast for 10 min a bit longer if it needs it.
then add some lemon juice to a cloth (real squeezed lemon) and wipe it round to make it smell lush.
Another good similar simple way is to put a bowl of water with a splash of washing up liquid in to the microwave and turn on full power for 4mins. Everything will wipe away with no scrubbing involved. Easy Peasy.
another good way is cut a lemon in half, rub the inside of the microwave then wipe off. you can use the other half to boil in water to make the microwave smell nice.