At Mexican markets throughout the West, freezer cases full of familiar shapes but not-so-familiar flavors tempt young (and young-at-heart) shoppers. They're paletas (little shovels, in Spanish), frozen treats that take ordinary juice bars to imaginative heights. Some are creamy, made with sweet tropical fruits and berries. Others are icy, flavored with tart fruits or even cucumber and chili (watch out ― the latter pack some heat). All are refreshingly low-fat and a cinch to whip up at home.