The microwave really needs a washing. This is what happens when children heat food until it explodes. I won't use cleaners that are full of chemicals because I don't want those fumes seeping into our food. Here's what I do instead.
I remove the tray and give it a wash in hot soapy water. (I love Mrs. Meyers dish soap!)
Next, I slice a lemon in half and squeeze it into a little microwave safe bowl of water.
I place it into the microwave and cook it on high for 4 minutes. The lemony water boils and steams.
When it stops, I don't open the door for 10 minutes, allowing that citrusy steam to loosen up the grime.
When I open the door, I wipe everything down with a clean cloth, and just like that, it's lemony clean.