This looks an interesting offer....

FreeClick kit (portable hotspot with SIM) will be available starting June 1st. 2014 up to September 30th 2014

There's a brief video explaining the deal here:

[video width=425 height=344 type=youtube]WAybwMemkJI[/video]

And the website seems quite informative FreeClick is a TOTALLY FREE high speed (21Mbps) Internet service for tourists staying less than 15 days in Spain.

No idea if it works but may be worth a try?