I'm partial to sangrias that have the extra kick of hard alcohol (e.g. whiskey, vodka, gin, brandy, rum). Lemon and orange slices are a must, of course, and you can add any other juicy in-season fruit you want. Keep the proportions as follows and you really can't go wrong:
For each bottle of red wine, add: 2 shots liquor (maybe tequila since it's Cinco de Mayo) 2 cups club soda, tonic, ginger ale, or any flavor of fruity soda (Hansen's works very well) 3 citrus fruits, sliced thinly (more orange than lemon or lime usually tastes better) 1-2 cups of other fruit (try kiwis) 2 tablespoons sugar (or less if the added carbonated drink is already sweet)