Mileage Plus members can now bid miles for new travel experiences and big-ticket items through the program’s new online auctions. United’s foray into the world of miles-for-stuff (preceded some time ago by Continental OnePass, which has been auctioning items for miles for years now) offers such goodies as a shark-diving expedition, a dinner party prepared by a gourmet chef, a relaxing massage, or a new Chrysler Crossfire.