Hi Mongoose,
You'll find lots of info on the La Gomera within the 'Other Canary Island' forum but you'll also find a wealth of information on:
http://www.gomera.info/
including links for the ferries and local buses. Usually I would suggest that you get the 8.45am Armas ferry from Los Christianos (LC) - slow, old and lumbering but such a nice way to start the day sat on the open deck at the stern soaking up the morning sun and watching Teide as it recedes behind you. It takes about an hour to get to San Sebastian (SS) which means that you'll have time to have a look around and/or have a coffee at one of the beachside cafes before getting the 11.00am bus over to Valle Gran Rey (VGR) in the southwest of the the island. This will take you right over the top of the island and enable you to see the amazing range of scenery and plant life to be found on the island depending on the altitude. It would be worthwhile breaking the journey at the top of the Valle Gran Rey Barranco at the Caesar Manrique Restaurant and miramar (viewpoint). There is a small garden area but the best bit is the view from the restaurant and it is a very reasonably priced place for lunch. From here you can walk down into the valley (about an hour at a brisk pace following your nose downhill all the way), wait for the next bus (but probably not until 3ish) or ask the staff to call you a taxi to take you down to the coast (about 15 euros last time I did it). Either way you are aiming to be down at the harbour in the Vueltas area for the 4.00pm local Garajonay Express ferry back to Los Christianos. The only problem with this is that it might stop running in mid-May, put out of business by the extension of the Olsen car ferry which is due to extend its service to here in late May/early June and for which timetables haven't been published yet. So before embarking on this itinerary do check the ferry timetables.
If by the time you go, the Olsen ferries are serving Valle Gran Rey, I suggest that you do this in reverse, that is get the first ferry of the day from LC to VGR (which will probably be the 9.00am). This will give you plenty of time to explore the Vueltas end of VGR and walk right round the bay to the La Playa end in time for the 2.00pm bus back to SS. If you want to break the journey at the mirimar as above, it makes a great place for a late lunch and the next bus - 2 hours later - would still get you back into SS for 6.30pm bus back over to LC.
Another great but alternative itinerary would be to get the Armas ferry over to SS and after exploring SS, get the11.30am bus to Vallehermosa in the north of the island. Again it will take you right over the centre of the island but the north is much lusher with acres of terreced banana plantations. As far as I know the bus still goes right down to the bay in Hermigua which is where I'd get off rather than going to Vallehermos. At Hermigua you'll find a nice little open air swmming pool (the sea is too dangerous for swimming here but note that the pool is shut on Mondays!) from which it's a short walk round the headland to the Castille del Mar which is a restored and converted banana export station which is now run a as cultural centre. The setting is spectacular and they do good tapas on an open terrace overlooking the sea. A leisurely lunch and swim should still leave you plenty of time to get the last bus back to SS (it currently leaves Vallehermos at 4.00pm) in time for a 6.30pm ferry back to LC. You have the choice of the fast Olsen ferry or the slow Armas one, which would be my choice. I can't think of a better way of ending the day than sat on the the open stern deck with a cold drink in my hand watching the sun set over La Gomera.
Whatever you decide to do, all the ferry and bus companies are notorious for changing their timetables at short notice, particularly in the Spring and Autumn so do check these once you arrive in Tenerife and before you commit yourselves. As far as I'm concerned, spending a night on La gomera is no hardship - we go for a week every autumn - but you might think differently if stranded there without your toothbrush
SM
PS There is no open air deck access on the Olsen ferries so the Armas ones are better if you want to stand a chance of seeing dolphins or whales on the crossing.