For a quick freshen up I wipe the insides with a vinegar/water mixture and then let it sit in the sun for a day or so ... sunshine has UVA and UVB which are disinfective and antimicrobial.
For a full-on cleaning, I use a handheld propane torch (from the hardware store) and lightly torch the inside. I still solarize for a day or so if I have the time.