from Rome to Siena you can get a train or a bus. If you book a bus in advance with SENA (bus company) you can even get a very low price (10 euro). Check it out here https://boa.busbookingline.it/web/eurolines/application/booking/seekBookingFrame.php?PHPSESSID=7bf870c63684803f7230075f4bf444e7&AZIENDA=SENA&cittaPartenzaDesc=ROMA&partenzaDal=08/06/2011&cittaArrivoDesc=SIENA&submit=Avvia%20Ricerca%20» (after finding time tables, click CHECK LOW COSTA AVAILABILITY BUTTON AT THE END OF THE RESULTS!)
Then to go to Cinque Terre you better get to Florence and then get a train to La Spezia. It's the easiest and cheapest way.
If you need accommodation in Florence, contact me!
Good luck, enjoy your holidays!
I've just realized that english version of the bus booking engine is not working properly in ENGLISH. If you can, try the ITALIAN VERSION.
To check train times and fares www.trenitalia.com almost 6 years ago