We love Venezia and think 5 nights gives you a great stay with 4 full days to enjoy! I would not do a day trip beyond the islands of the lagoon and Padova. Rick Steves' guide gives great detail on both. Spend a day hopping to Murano, Burano and Torcello, then rest up and walk Venezia by evening light after the cruise ships and daytrippers have left.
Rick also suggests a couple of self-guided walks, but for more inspiration, you might like the book "24 Great Walks in Venice," which offers routes throughout Venice and gets you out of the main tourist tracks.
I would buy a vaporetto pass for a few reasons:
easy to hop on when you are tired even if just for a couple of stops
for the lagoon islands which will cost you dearly per person without a pass
access to the bell tower at San Giorgio Maggiore, across from San Marco, a better and less crowded view than from the one in San Marco
to be able to cruise the Grand Canal just for fun
Here's a fun restaurant list to try. We really liked Ai Artisti.