I love New Year's Eve in Florence - you just walk around from piazza to piazza, listen to music, people watch - it is just a great big street party - and then at midnight people get a little crazy and smash bottles and light fireworks... it is a little nuts but usually manages to stay under control and people are happy and friendly. Here is this year's line up of music in the various piazzas. We usually end up in Piazza Signoria for the bottle smashing and crazy people running through fireworks :)
There are also many, many places (fancy restaurants and the hotel restaurants like the St. Regis, etc.) that have prix fixe dinners with everything included if that is more what you are looking for.