When Traveling I find it quiet common to kasher a microwave to use for cooking. Many apartment type hotels will have a microwave and often families I may be staying at who don’t keep kosher will also have a microwave.
Here is some information from Halachfortoday.com about koshering a microwave. It is a site that sends a halacha to your email every day which I highly recommend)
Someone warmed up meat in a milchig (dairy) microwave can I Kasher it?
What about the trays?
Yes, a microwave can be Kashered, as follows:
It should be totally cleaned. A cup or bowl of water should be placed inside and the microwave turend on for approximately 10 minutes. The bowl with water should then be placed in a different place in the microwave, and the microwave should be turned on for an additional 5-10 minutes.
If the microwave is being Kashered from a non Kosher usage (as opposed to just from milk and meat) 24 hours should be waited before doing the above procedure.
If there is a glass plate in the microwave, a Rav should be consulted regarding if it is able to be Kasherd, and if so, how. (See Piskei Teshuva Orach Chaim Simon 451 for more on this toipic)
In general, it’s important to remember that anytime something dairy is warmed up/cooked in a fleishig microwave (or vice versa), it should be covered well with a double wrapping.
If a Chometz microwave is being Kashered for Pesach, the food warmed up in it should be double wrapped, even after Kashering.
If there are any plastic components on the inside surface of the microwave, a Rav should be consulted to determine if it is possible to Kasher.
If the microwave has a browning element or is a convection oven, it needs to be cleaned, and then turned on to its highest setting for 45 minutes, in order to reach its highest heat and thus render the oven Kashered.
There are certain models that do not reach the required temparature to effect a proper Kashering, so it’s importantto check with a Rav regarding your particular model.