Easy Expat's guide to Madrid offers some great info about summer/seasonal jobs in Spain: http://www.easyexpat.com/en/madrid/job/jobs.htm

i.e.
Camping agencies in Spain offer jobs during the peak tourist times in the summer.

Eurocamp (http://www.eurocamp.com) is the market-leader in self catering holidays to Europe on holiday parcs accross Europe. They often need for couriers on Spanish campsites. Employees must be over 18, and can anticipate earning approximately £95 per week.

PGL (http://www.pgl.co.uk) is a company that organises holidays for children and summercamps can be found in UK, France and Spain. They operate from Easter to October and during half terms as well.
Working for PGL as a helper or courier can earn you between £50 and £60 per week in Spain and board and bed is provided.

EuroSummerJob [http://www.eurosummerjobs.com] provides more than 1000 summer jobs offers throughout Europe.

There is also a new article on WWOOF which a unique opportunity to work on organic farms in exchange for room & board: http://easyexpat.blogexpat.com/blog/exp ... -the-world

Enjoy!