Does one need to purchase tickets for the various sites in Venice? St. Mark's Basilica or the Doges Museum? Is there a package?
There is are packages sold by Venezia Unica.
http://www.veneziaunica.it/en/e-commerce/services
Look through and see if any are worth it to you. Depends on how many you want to see as you get one entry per site on the pass.
Go to Correr Museum inside the square itself, opposite San Marco. Buy a combo. pass. This will get you into the Correr museum, the Doge complex (and you can skip the ticket buying line and breeze through the reserved ticket line), and the Mariani Library (across from doge complex). I don't remember San Marco charging for entering the church. There are fees for going to the upstairs museum and balcony. Also, fees for the inside "treasure" exhibit and the gold altar piece palo doro.
You pay for those inside the church at each point of entry.
St. Mark's Basilica is free to enter and you can pay inside for an extra little museum; if you want to reserve an entry time online instead of standing in line, that's €2 for the reservation. The cheapest combo ticket that covers the Doge's Palace is the one that pairs it with the Correr Museum for €18 (sold online or at either location).
The museum on the second floor of St. Mark's is great because you can go on the outer balcony and get a great bird's eye view of the square. Well worth the 2 euro fee.
There is also the Secret Itineraries tour of the Doge's palace and that should be reserved in advance. We thought it was interesting but it sometimes gets mixed reviews. It goes into some attic spaces and could get hot during summer months.