The microwave is one of those appliances that is easy to forget to clean. Since the door is almost always closed the nastiness that lurks inside can go unnoticed for quite some time. Luckily the microwave can be cleaned in about 5 minutes with only plain water and very minimal effort. Even if your microwave is like mine with gross caked-on food seemingly baked into the walls, I promise you this will only take 5 minutes.
Take a microwave safe bowl and fill it with water. Add a dash of vinegar and a few squeezes of lemon juice. Set it in inside and turn the microwave on for 5 minutes. The water will turn to steam and the steam will soften up all the gunk. The vinegar helps to absorb the smells.
While it's cooking give the outside a quick wipe down and then go do something else for 4 more minutes until it's done. You can let the microwave sit for a few more minutes to let the steam penetrate even more if you like.
Be really careful when opening the microwave and taking the bowl out. The water will be extremely hot. Wipe down the walls with a damp cloth and remove the plate from the inside. Rinse it in the sink with hot soapy water. Put the plate back in and voila, a nice squeaky clean microwave ready for the next person to cook something and forget to cover it!