I agree with the lemon and water and let it boil in there then wipe down the whole microwave. We just had burned funnel cake in there and it mostly removed the smell but the wiping off skills of my daughter...
A bowl of cat litter? (is supposed to absorb some odors)
Some of that citrus-scented cleaner?
Back when microwaves first came out my uncle put a baked potato in and set it for 40 minutes. (for some reason he didn't get that microwaves were supposed to speed up cooking....) He said when it was done it was just a little black husk and a bunch of smoke...