Travelling Bilbao to Grenada

I don't like to fly unless I have no choice, so I would look at the train options first on the Renfe website.

There is also one train a day at 7am, but it requires a transfer in Madrid from Chamartin station to Atocha station, total journey time around 9 hours.
Alsa Bus has a departure daily at 0900, transfer at Madrid, journey time 10 hours 30. (and also an overnight departure at 2am!)
Assuming you mean Granada, rather than an island in the Carribean!!

So no wonderful land alternative, but then driving would take 8 to 9 hours without stops- which makes bus and train quite reasonable.

My workaround for this sort of situation is to develop a burning desire to see one or two places along the way and spend one or more nights in transit. (Yes, I'm retired.)

I hate mid-trip flights, but spending an entire day on trains or buses isn't something I want to do unless I'm traversing a lot of fabulous scenery. That's not going to be the case on this trip.

Booking tickets on one of Vueling Airline’s direct 90-minute flights from Bilbao to Granada is an option.
www.Skyscanner.com has flight schedules and fares.

acraven-great answer!

Thanks everyone for your suggestions. I think we will be flying. Seems like the best option.