Venice needs 3 nights, especially since your first day is jet lag day. Use one of the days to visit Murano/Burano.
You don't say what your interests are but you can't go wrong with either Rome or Florence.
Where are you flying from?
2 nights really equals 1.5 days.
You have 11 nights and you want to be in the city that you fly home from at end of trip.
If you want Amalfi coast I would suggest:
Venice 3 nights
Train to Salerno- we are taking direct train in opposite direction so I assume you can do the same- it's a 6 hour train ride but gets you to Amalfi without changes in Naples/Rome, etc
From Salerno you can take a ferry- right near train station- to Amalfi.
Stay in Amalfi as it is transportation hub for all Amalfi sites
Amalfi- 4 or 5 nights- day trips to Capri, Positano, Ravello, Pompeii if you like (or maybe on your way to Rome)
Rome- 3 or 4 nights- not near enough time for Rome but gives you a taste.
I like to use rome2rio.com to scope out transpo options.
www.rome2rio.com
ETA-- just saw your previous post- gotta be honest- if this is your first trip to Italy I'd do
Venice-3
Florence-4 - day trip in Tuscany
Rome- 4
The beaches in Amalfi are mostly rocky from what I understand, I just don't think I'd spend my first trip to Italy sitting on a rocky beach.