What do you want to say with amateur races?... It´s like master30, master40 races or something like that, or it´s something like Quebrantahuesos, Lagos de Covadonga,... cyclotourist races.
If you are looking for the second kind of races, you can look the www.altimetrias.com website. There you have the information about the most popular amateur races, and also the dates and profiles info. (look in the left side, Marchas Cicloturistas 2012).
http://www.altimetrias.net/marchas/marchas.asp
You say that you are going to stay in Spain, but Spain is so big to go from Andalucia to the north (Asturias, for example) in one day and go back.
Give me some more information, and of course, sorry because my english is terrible. I´m really apologized about it
P.S.: For me, the most beautifull cyclo race is Irati Xtrem. Is not a competitive race (only you can check your time in the Port of Larrau), but it has everything to enjoy over the bike. Terrific "cols", agressive descents and a very good atmosphere.