No one answer is possible, so here are thoughts. First, Venice has an uncanny ability to tell you what to do. The space is so limited that you could easily spend two days wondering around the “calle” near St. Mark’s and Rialto. Even random “vaporetto” rides are great fun. But if you want something that (ironically) celebrates life, visit the San Michele cemetery. Apart from seeming like parts of New Orleans for its beauty, lushness, and hail-fellow-well-met approach to death, you’ll find the tombs of Stravinsky, Diaghilev and Pound. It’s deeply moving.