Day Trip to Cinque Terre from Siena -- via driving to La Spezia

Drive time about 2.5 hours each way. You can park under La Spezia Centrale station in a nice safe lot.
That is a lot of driving for one day, and it's not easy, especially the late night drive through the hills back to Siena. Consider driving to Sestri Levante and staying over one night. It's a nice town and your whole day visit to Cinque Terre will be much more enjoyable.

There are more parking options in La Spezia and the larger towns than in the five official Cinque Terre towns, so it does make sense to park "outside" and take the train in, rather than driving to a small-town parking lot that may be full. To get between the towns, you'll can take local trains or boats, or hike. If the La Spezia station parking lot is full, there's also a free lot further away at Piazza d'Armi (see pages 491-2 in Rick's Italy book). The best time to arrive is as early as possible, so you can enjoy as full a day of "relaxing" as possible.

There is a multi story parking structure next to the La Spezia station. That is assuming you want to deal with the city traffic of La Spezia.
There is also a parking structure at Riomaggiore, but to get there you still need to drive through La Spezia. Also in August there is a high chance you'll find it full (don't go on weekends, at any cost).
Another option is to stay on the A12 freeway a little longer and go around to Levanto (exit at Levanto). Drive to Levanto and from there to Monterosso, where there is plenty of parking at the lot on the beach. Coming from Tuscany, it's a little longer in distance compared to La Spezia, but probably not in time, because you'll be avoiding driving in the city altogether and you'll be parking right at the Cinque Terre (Monterosso).
Avoid going there on a weekend or you'll be stuck in traffic all the way along with all the Florentines trying to get to the beach and back. The A11 (Firenze-Mare) on summer weekends is bumper to bumper and the average speed is 0.0001 km/h (on the fast lane). The SGC - FI-PI-LI motorway via Empoli-Pontedera is just as bad.

Thanks everyone! Sounds like it might be a lot of fuss and muss (re: driving/parking etc) for just a short day visit to CT... especially in August when it will be teeming with tourists there! We are spending 4 nights down in the Amalfi Coast on this trip, so now I'm considering skipping CT altogether since we will have some "coast time" down in Sorrento anyway. So, instead of CT, perhaps we should spend that day visiting some of the hill towns in Tuscany? Our base is Siena for 4 nights. We plan to be in San G for a half day... any other small hill towns in that region that are a must-see? Thanks in advance!

Yes, skip the Cinque Terre. The Amalfi Coast (Positano, Amalfi etc.) and the islands in the gulf of Naples (Capri, Procida, Ischia) are just as beautiful.
There are lots of options near Siena: San Gimignano, Volterra, Monteriggioni, Certaldo, Colle Val D'Elsa, Pienza, Montepulciano, Montalcino, San Quirico, and hundreds more. Even Cortona and Assisi (the latter in Umbria) are within 1.5 hour drive from Siena. Check the website below for ideas. Any towns in any of the provinces of Tuscany are within 2 hour drive.
www.borghiditoscana.net/eng/

Thanks Roberto! We will plan to spend that extra day exploring the region surrounding Siena instead. I also just read somewhere that portions of the hiking trails between the CT towns are still closed -- wonder if that is just typical in the spring or is this also the outlook for the trail conditions this summer? Just curious!

momto3,

Most of the Sentiero Azzurro (formerly called No. 2) trails do appear to be still closed, although I believe the trail from Monterosso to Vernazza is currently open. There's an extensive network of trails in that area, and most of the longer ones that go up into the hills are fully accessible.

The trail from Monterosso to Vernazza is the most challenging of the four. If you're planning to hike that one, I'd suggest getting started early if you'll be there in August as it will get very hot later in the day. I prefer to start at the Monterosso end when I'm well rested, as there's quite a climb at the beginning. Be sure to take lots of water (I always take extra so that I'll be able to leave some for the stray cats at the Vernazza end).

If you decide to stop even for a quick look at Monterosso, I believe there are two routes into town, one that comes in at top end of the "old town" and one that goes to the end of the new town (which is where the parking lot is located that Roberto described). Hopefully Roberto or one of the others can clarify the driving directions. You will NOT be able to drive between the two parts of Monterosso, so be sure to take the correct exit.