I agree with Jessie -- heading out to the islands in the lagoon is a wonderful way to spend the day. It wasn't as crowded as I expected it to be in June -- both Murano and Burano were fairly quiet. After waiting in long lines in Venice, it was lovely to head out to the other islands.
While in Burano, which is a must see, IMO, -- the colors on that island are so fabulous, take the time to walk the bridge that links Burano to the tiny island of Mazzorbo. I imagine that Mazzorbo is, today, what Burano used to be before it was rediscovered by tourists. There is an interesting modern housing development built on the island, along with a crumbling grand-dame of a church, Chiesa di Santa Caterina, which, adjacent, is a cemetery that is worth the visit. You could spend less than an hour in Mazzorbo and really see it all.
Also highly worth the time is a stop in Murano. The glass, obviously, is the main attraction there. I visited the Formia Factory and had a great time watching the artists shape the glass. If you like glass, you may be interested in seeing the
Barovier & Toso Museum. This particular factory has a very long history and is quite fascinating. Most of the factories tend to close up for lunch, so keep that in mind as you are walking around.
Also great about Murano are the huge glass sculptures that dot the island -- all are quite modern, which makes for an interesting contrast between the very old churches and bridges. I found some really great bargains on beads, too, so if you have an interest in buying real Murano glass, do so while you are there. You'll see big mark-ups in Venice for the same stuff.
This might sound kind of cheesy, but you don't necessarily have to go to a bar to find good night life. I've had many fun evenings sitting in a piazza with a bottle of beer or two, just watching the locals and the other tourists. This can be especially fun if you find a spot where there is a game going on. Often, bars/restaurants that face a piazza will bring out their tv's to watch a soccer game (the waiters spend as much time watching as do the patrons, probably even more, actually). No one seems to frown upon public drinking, either. I just stopped in a local deli to buy beer -- I think I spent about 6 Euros for two big bottles of beer.