Drivers license

The DLis not a bad idea. Sometimes the museums, exhibits want you to leave an id, passport, or a credit card for the headphones. The DL is the least risky thing to leave.

Yeah, I always take my DL too. If your not taking a limo to the airport you'll need it in case your stopped.

I always take mine and I agree with Frank leave it for hiring an audio guide or similar. To me I would prefer to loose my driving license rather than my passport. My passport is far more precious.

Just don't do what I did in Lucca and leave it behind! I gave it as id to use the internet at the TI, the assistant was busy as I left, took my money but didn't give me my licence back and I didn't realise until it was too late. Of course the next day when the TI should've been open it was shut for some reason. "Luckily" I became ill and we had to stay another night and we picked it up on the way to the train. Actually it was in the complete opposite direction to the station, but anyway . . . I often carried it as id when I didn't carry my passport, but that's been covered numerous times.

Ditto to what everyone else has said. It's always a last resort option if something happens and you need another photo ID or find yourself without your passport for any reason.