Hi SEAK
It will depend on where you and the rescue center are. Find out what your target audience is, and how big.
You would need to check out the legalities in spain, but a few ideas could be: -
Raffle tickets - Christmas will soon be here
Open day for a price -free wine etc.
Dog training
Set up a small web site - how about dog/cat adoption - pay 2 euro a month to receive information and pictures etc.
Find a popular music venue - see who pulls the crowd, use them to organisize a dance
Curry evenings, Bingo - etc
The list is endless
I have played in bands for 40ish years, and have raised 1000s for charity
Its all a matter of recognising the opportunities
Hope this gives you some ideas
All the best
Phil