raz - looks like a good plan.

ref your 'day trips", in Venice you really only have time for a trip to the islands [I suggest Burano and Torcello], in florence a bus ride to Siena would be my favourite, and in Rome, a day trip on the train to OrvietO.

the beauty of all of these is that you don't need to decide in advance - just do it on the day. the caveat is that if you go to the islands when you are in Venice, the individual tickets are just about equal to the cost of a vaporetto pass for 3 days, so even were you to walk everywhere else, it's worth having the pass just for that.