Fisterra, Muxia, A Coruna, Ferrol, Lugo, Ourense: they're all nice places to spend a bit of time. All accesible by public transport and all as much fun as they can ever be in the cold and the rain. That said I've sat in a hot spring in Ourense in a snow storm. Getting out was the troublesome bit. In your position I think I'd spend enough time in Santiago to really appreciate the place, maybe three or four days. Then I'd head to Madrid: between the Prado and the Thyssen-Bornemisza and the C. de las Huertas and the Parque de El Retiro and just sitting outside a bar around the Puerta del Sol, well: a chance to spend some time in one of Europes great capital cities