Of the many possible ways of saying it, I think this would be my choice.

First, include "credits", since your question makes it clear that this is what it's referring to.

Generally speaking, they're called "free choice" in the UK and "elective" in the US. "Free elective" is not widely used; it's found in the English versions of Spanish university websites but it's a bit of a tautology; electives are free by definition.

I think "por" is best done simply by "for"; that is, "awarded for doing".

"Credits for activities" alone is sometimes found in genuine British and American academic websites; for example:

"The Sheffield Landscape Graduate Award allows students to accumulate credits for activities that they contribute to outside their course."
https://www.shef.ac.uk/landscape/study

However, "activities" alone sounds a little strange to me; it seems to need something else. You sometimes find the expression "elective credits for activity courses" in American sources, but I don't think that's suitable, because these "actividades" are not courses, by and large. What they are is extracurricular activities, and that's what I'd be inclined to say. They can include a whole range of things, such as sport, work placements, mentoring students, work in university associations, and so on. Here's a document on "Normativa para el Reconocimiento de Créditos de Libre Configuración (Libre Elección) por Actividades" from the Universidad Politécnica de Valencia, which includes
1. Tutorías [mentoring]
2. Actividades en Empresas
3. Colaboración en centros, departamentos e institutos
4. Tipo del proyecto fin de carrera
5. Cursos, Seminarios o ciclos completos de Conferencias
6. Programas interuniversitarios, internacionales de movilidad de estu-diantes, suscritos por la u.p.v: erasmus, socrates, lingua, tempus, alfa, med campus e intercampus, y otros que puedan existir.
7. Idiomas
8. Deportes
9. Actividades Culturales y Sociales
10. Actividades de representación y asociacionismo universitario
http://www.upv.es/entidades/ETSINF/info/normativa_cas.pdf

So really, they are by definition outside the normal curriculum, and cover a range of things that are normally called extracurricular activities:
http://en.wikipedia.org/wiki/Extracurricular_activity