I tend to dress about the same when I travel (to Italy or anywhere else) as I do casually at home The clothes you describe sound fine to me for Itally.
I'm not sure what's on the Italy tour in Venice One thing you could do - if it isn't on the tour - is take a vaporetto boat out to the island of Burano, with its lovely colorful houses. The ride out there from Venice is not quick, though - with boat connections figure on an hour each way. I'm guessing Burano would be mentioned if it is part of the tour, so if it isn't mentioned, go for it.
Find out exactly what your credit card company charges for foreign transactions. Many credit cards charge 3%. It is easy to get one that charges 0%, but it might be too late to get one for this trip. Or it may not be worth it. At least find out what your credit card company charges.
Also find out what your bank or credit union charges for ATM withdrawals. You again might be charges a currency conversion fee of up to 3%, PLUS a per-use fee at the ATM. It's again possible, with enough time, to get an ATM card that charges 1% conversion fee (or even 0%) and $0 per use. The ATMs in Italy usually don't charge anything - it's the ATM at home that might. Find out what it will cost you. Then, you can figure out whether it's better for you to use your ATM card and pay with Euros in cash - or use your credit card.
One tip: ALWAYS pay in Euros in Italy! That is, if you pay for something with a credit card and you are offered to make payment in US Dollars...DON'T DO IT. This is called Dynamic Currency Conversion (DCC), and it's just a rip-off designed to charge you even more. Just pay in Euros (same thing at the ATM) - decline any offer to pay in US dollars. The Euros will be converted to dollars for you.