Venice, Italy

If you can only do one thing (or two):

Go to the Hotel Danieli, its just past the main square. Take a left, cross a tiny bridge and then its on the left side. Go into the lobby, and there is a nice lobby bar, but go straight past it, past the sign that says hotel guests only, take the elevator up to the rooftop terrace. Super expensive drinks, but the view and peace and quiet are worth it. Great escape from the tourists. 

You'll want to visit St Marks Basilica, its the main attraction in Venice, and its free, but there was an HOUR line to get in. Forget waiting in that line- go to this website, choose individual traveller, and its 2 Euros for ZERO wait time. There is an entrance in the front of the building, no line. You show them either a printed confirmation from this or just a barcode on your phone. 

http://www.venetoinside.com/attraction-tickets-in-veneto/tickets/skip-the-line-saint-marks-basilica/

That website has other skip the line tickets etc. for other sites in Venice too and I believe that they have deals on taxis etc.