I would like to do a day trip to the beach during my stay in Italy and am considering staying an extra day in either Florence or Rome. But which city has the best beach near it? Florence or Rome?

I saw that Viareggio is the closest to Florence and Ostia Lido is near Rome. Are there other better beaches? Which do you think is the best?

I would go to Sperlonga, a train ride south from Rome. Ostia Lido is pretty trashy.

Is the typical weather in mid May suitable for the beach?

There hasn't been "typical weather" pretty much anywhere on the planet recently IME. That said, I might go to the beach in Italy in mid-May, but I wouldn't expect to swim.

I've been to Ostia and StCirq described it well - - it is not that fun to access, and doesn't feel all that special. I've been to Viareggio and the beach is a long walk from the train station, but is otherwise perfectly o.k. The town is generic and uninteresting. Forte di Marmi, just north of Viareggio, is supposed to be more upscale. Have a look at them on Google images. StCirq's Sperlonga sounds intriguing, and the town of Sperlonga itself might make it an attractive option.

Hmm. I think I'd forget the beach and go to Siena for the day!

For Sperlonga, you might have a look at...

http://www.tripadvisor.co.uk/ShowTop...ome_Lazio.html

(If it's not clear from that, the station is shared with nearly Fondi - and actually called "Fondi-Sperlonga". Tickets for the 70 minute rail journey are currently €6.20 each way, then it's a Euro or two on the bus!)

Or jump straight to some photos from a couple of June visits we made a good few years ago, with...
http://www.pbase.com/isolaverde/slides_sperlonga

We now live 40 or so miles to the south and, although I've no crystal ball to offer, you'll perhaps get an idea of how the last six or seven Mays have been from these...

http://www.pbase.com/isolaverde/mayisch

Peter