If you book independently, you may risk not getting back in time.. When the ship sails...be on it.
The Parthenon sits atop the Acropolis and is a the most important structure on the planet for what it represents, is also a must see. The view down onto the incredible city of Athens is only realized by being there. It is memorable.
Use a water taxi (Vaporetto) to get around.
San Marco is the stop with most of those trademarked sights.
St. Mark's Square is here as well as Gondoliers and narrow canals from which paintings are made.
Cafe's and shops round out a perfectly delightful day. Once you are jostled by a pickpocket, it's too late. The handoff has ALREADY OCCURRED. Do yourself a favor and purchase a Civita Pocket to put inside your trousers. It really provides peace of mind (for the entire trip) Venice can be quite crowded on a sunny afternoon.
Venice is gorgeous by anyone's accounting. You'll remember it forever.
If your waiting to embark on a cruise ship from Venice and will use a hotel in the meantime, may I suggest you consider paying up to double for a hotel in Venice rather than a hotel out of town that is accompanied by a 150 Euro round trip taxi ride like we did.
Venice is a must see. You can easily do this on your own..