Daytrip to Bilbao from Barcelona?

Gee you would want to be keen to visit Bilbao. Primarily, I guess to see the Guggenheim. It is spectacular, but for a day?
I don't think the 2 day option (including San Sebastian) is feasible as there appears only one direct flight per day (in the early evening).
Remember you will lose about 8 hours of the day travelling with checkin, flight and airport time.
That eight hours could be used well in Barcelona for further sightseeing (or a day trip to Girona & Figueres).
These are the things you have to weigh up.

There is enough in Barcelona and it's environs to fill your time for weeks. I'm not sure why you would spend 5-6 hours commuting by air just to see Bilbao. And it will consume 5-6 hours minimum of your day just getting to the airport, security, boarding, flight, transit into the city and doing it all again on the return.

If you need daytrip ideas near Barcelona, let us know. There are a multitude of options for all different tastes and most with public transit options.

We've been to both. I love San Sebastian, Bilbao and that whole part of Spain, largely because it is so different from the south. It is worth more than a week of exploration on its own.

You have a very short time in Barcelona. Stay there and enjoy it to the hilt. And if it is your first trip to Spain, remember that meal times are very different from what you may think is normal: http://spanishfood.about.com/od/discoverspanishfood/a/spain_meals.htm, http://www.totallyspaintravel.com/2013/10/31/when-should-you-eat-while-in-spain/.

Thanks for the input, everyone. Douglas, what are the good day trip options that aren't listed in Rick's book? (i.e., we know about Monserrat, Figueres, Cadaqués and Sitges). We like nature and, unless it's something spectacular like the Guggenheim, are not really much into museums for this trip. We have traveled a lot in Europe and been to Madrid, Toledo and southern Spain. We are looking forward to the architecture in Barcelona.

BTW, there are plenty of flights to both Bilbao and San Sebastian on Skyscanner. Our Barcelona apartment is 30 minutes from the airport. It looks like taking a morning flight to Bilbao, going to the Guggenheim for 2-3 hours, heading to San Sebastian for an early evening pinxtos tour and flying back to Barcelona late the next afternoon after a day of wandering San Sebastian and learning about the Basques, would be a nice complement to our 3 days of sightseeing in Barcelona. It's always good to be reminded of the logistics, however, and we will carefully consider the timing of it all.

I recommend 4 full days in Barcelona alone to do it full justice (and this does not include Parc Guell)

DAY 1 Ramblas & Bari Gotico
Metro to Placa de Catalunya, Barcelona’s main square
Explore the Ramblas from top to bottom taking side diversions
On the right C. d’Elisabets to the museums MACBA and CCCB (check for current exhibits)
Further down to La Boqueria markets
Left side thru the Gothic Quarter via Placa del Pi & Placa de Sant Felip Neri to the Cathedral
Placa del Rei and the wide Placa Sant Jaume flanked by official buildings of the city and state
Return to the Ramblas (at the Liceu opera house) down the shopping street C. De Ferran
Further a little to Placa Reial, harmonious enclosed square with palms trees & Gaudi lanterns
And then Palau Guell (Gaudi) off on the right
Finally the port and the column with Christopher Columbus gesturing towards the new world
Metro Drassanes is close by

DAY 2 Montjuic & Olympics
Metro to Placa d’Espanya
Les Arenes, once the bullring, now housing shops & restaurants (fighting is banned in BCN)
2 large towers opposite and the Av de la Reina Marina Cristina towards the Palau Nacional
This area of exhibition space was the site of a Universal Expo in 1929
(Barcelona had Olympic visions way back then but the Spanish Civil War put them on hold)
Caixa Forum and Mies van der Rohe Pavilion are both worth a look
Escalators to the Palau and the MNAC museum, well worthy of a visit
The Anella Olimpica (Stadium, Palau St Jordi, Communications Tower) is laid out behind
Now back down the hill to the excellent Miro Museum (Fundacio Miro)
Piscina Municipal close by – spectacular Olympic diving venue
Along Av. Miramar to the Funicular connecting with Metro at Parallel

DAY 3 Gaudi & Modernism
Metro to Sant Pau to explore the modernist hospital complex of Sant Pau
Walk down the Av de Gaudi to Sagrada Familia (Gaudi) – PREBOOK TICKETS !!
Metro to Diagonal and Passeig Gracia for Casa Mila (Gaudi)
Walk down Barcelona's majestic boulevard to crazy colourful Casa Batllo (Gaudi)
Casa Amatller is next door and the Loewe store on the corner is in Casa Lleo i Morera
Finish at Gran Via – Metro Passeig de Gracia is closeby

DAY 4 El Born & Barceloneta
Metro to Arc de Triomf
10 min walk to Palau de la Musica Catalana – PREBOOK TICKETS !!
Through the revitalised area of El Born to the Picasso Museum – PREBOOK TICKETS !!
On to Santa Maria del Mar and then into the Parc de la Ciutedella for a relaxing circuit
Past the historic railway station, Estacio de Franca to the old fisherman’s village, Barceloneta
The ‘D’ shaped W Hotel stands at the port entrance
Walk the other way along the beachfront towards Frank Gehry’s glimmering ‘Fish’ sculpture
Finish at the Hotel Arts and Port Olimpic – Metro Ciutedella / Vila Olimpica is close by

You asked about daytrip options from Barcelona. I loved Girona. My guidebook at the time called it "a world class city, patiently waiting for the world to notice" and they were 100% right. When I went in 1995, it was about 90 minutes from Barcelona by train or bus. But, Barcelona resident Enric has posted that there is now an AVE (high speed train) and it only takes 40 minutes.

Daytrip options depend on whether you have a car or not. Girona, Montserrat, Figueres, Sitges, and Tarragona all are easy to see by train with no more than about an hour's transit. Tarragona is one of the oldest Roman cities in Spain and still has a dramatic coliseum along the sea, a great archeology museum, a Roman wall, ruins of the chariot stadium and a few other minor ruins. You can easily spend a day there.

With a car, you can drive to Montserrat and then spend a few hours tasting cava. Or drive up into the foothills of the Pyrenees to places like Besalu with stops in Girona or Figueres. Or spend a Dali day visiting Figueres and then Cadaques and Port Illgat. Or drive north along the coast and visit the seaside towns.