Honey, load the Winnebago! Proprietors hope the gargantuan Mall of America, opening in Bloomington, Minn., this week, will pass even Disney World to become the hottest vacation destination in the U.S. They expect 40 million visits in 1996. Besides its own amusement park, the mall will feature a Hormel cookout area -- SpamLand? -- and (move over, Epcot!) the LEGO Imagination Center, a 5,000-sq.-ft. room of giant LEGO models. Sorry, kids, you can't build this stuff at home.