Hi; we live in Bizkaia, my wife being Basque.
Depends what time you disembark in Bilbao. If early in the morning, you could go mad & do it to the costa del sol in a day, tho' I'd hate to do that & miss interesting parts of Spain.
Vitoria isn't much really; Logroño has good pintxos (tapas to southerners!!) & Laguardia in la Rioja is an interesting wine hill town. Problem then is that the national road to Soria is a bit twisty & then you're faced with probably going to Madrid & round one of its too many ring roads which confuse me like hell.
Otherwise Bilbao-Burgos on autopista, which'll be about €14-15. Countryside a bit boring, but Bilbao-Burgos you should be able to do in around 2 hours. Burgos catherdral worth seeing. Then autovia, free, to Valladolid & from there, you could do worse than see some of the more spectacular castles of Castilla, such as Medina del Campo, Coca (a real gem) Isabel's birthplace of Madrigal de las Altas Torres (no castle), & then Avila, where you could stay overnight.
From Avila on the N403 thro' the Sierra De Gredos to Toledo then on autovia to Consuegra (windmills), Valdepeñas, then south, perhaps stopping to visit the museum at las Navas de Tolosa, where the Moors were defeated in 1212 (museum closed Mondays). It's signposted off the autovia & is just north of la Carolina. Ubeda is an interesting hill town also to visit, just south east of Jaen. Once you're there, I guess you know the way better than me back to the Costa.
regards
paul attard