Hi,
It depends on what you want to see. Spain is too big to try to see all areas.
Probably the best birding site is Doñana National Park (South of Spain). The second best place that I recommend to see waterfowl is Delta del Ebro Natural Reserve (
http://ebre.info/), in Tarragona (Catalonia) (north-east of Spain). Near Barcelona, and 90' by car from Delta del Ebro, there is an small reserve (Delta del Llobregat); In this last, numbers of birds are low, but you can see them very near. Both Doñana and Delta del Ebro are good places also to go to the beach with the family.
Completely different is Extremadura (south-west of Spain;
http://www.turismoextremadura.com/), where you can see black vultures, eagles,...(specially visit Monfragüe National Park). An also some cities are really interesting, as Cáceres (medieval town), Trujillo, Plasencia, and Guadalupe, among others. They are all medieval cities. In all of them you would see lots of european white storks, and Falco naumanni. Obviously, there are no beaches there. Near Extremadura is the Cabañeros National Park, also very interesting, and Daimiel (wet area), that this year has water!!!
All center of Spain is very interesing to see vultures and eagles, and also to see some Otis tarda (I do not know the name in English, I am sorry), and other steppe birds (Tetrax tetrax,...). I recommend to visit Villafafila lacoons in Zamora (
http://www.villafafila.com/) and la Nava (
http://www.fuentesdenava.com/files/nava_global_nature.htm) lacoons in Palencia province (middle-north of Spain), where you can see also waterfowl if they have water. Both of them are in Castilla y Leon that has at least three cities that are very interesting also for the family (Salamanca, Segovia and Avila; all of them are medieval, with some roman ruins in Segovia). Dont forget some Natural Parks in Segovia (Hoces del Rio Duratón, near the village of Sepúlveda, and Hoces del rio Riaza near Ayllón), both interesting to see vultures, eagles, milvus species, and some interesting passerines, as Dupont's Lark (in Hoces del Rio Duratón). Obviously, no beaches there (except rivers), but any way very interesting for birding and culture.
I recommend you to visit the web of the Spanish Ornithological Society, where you can see some information on where to see birds:
http://www.seo.org/donde_ver.cfm
I also recommend to buy the book "Donde ver aves en España" (only in Spanish, I think) (
http://www.casadellibro.com/libro-donde-ver-aves-en-espana/1043679/2900001070931/en_gb) (author:Montero JA), or the book "Donde ver aves en Extremadura" (
http://dondeveravesnaturalezaextrem.blogspot.com/). Both have routes to see birds, one in Spain and the other in Extremadura.
I think that staying in an island (as Mallorca) is not the best idea to do birding. They have interesting species, with some specific of the islands, but always less than in the continent. If you need beach, choose some in Andalucia (near Doñana if possible), and then, from there, you can try to move to see other areas. In Andalucia you have also the Natural Park Los Alcornocales in Cádiz province (near Doñana), or some areas in Huelva province.
Enjoy it.