Hi! As I'm Spanish and I speak several languages, I'll definitely recommend you Portuguese. It's real fun! Also Italian or French but they are more difficult.
As you this, where would you want to go for holidays or which people do you think are more fun? That's your new language.
Well, I think it depends on you: on your interests and hobbies, on your goals and on your career. If you don't have a reason to learn a third language, you'll probably lose the will to keep studying it.
If you are willing to be a polyglot, maybe Italian, French or Portuguese. Being Spanisch your mother tongue, these languages are easy to learn. You'll get them very fast.
If you're an engineer or work with IT, maybe German.
As the colleagues said, for you as for Spanish native-speaker it would be quite easy to learn French, Portuguese or Italian, because these languages have much in common.
But if you would like to know some international languages that are used in our world more often and widely, i can recomend you German and Russian. In a package with English and Spanish it would be quite completed and useful group of languages that you could apply almost everywhere in Europe.
It is only up to you.
If you have a strong motivation, every new language will be a real treasure.