A small round cantine, that is strapped around its wearer with a leather strip. It has the ability to never run dry. It will be able to contain any type of liquid such as acid or lava. It will also never run out.
The PCs are sent with a diplomat to another country to protect him and do his bidding. There may be some espionage, rescuing, downright bullying, etc. Could make a nice medieval, special operations background.