I would probably not translate the word. Note that it is left untranslated in the original text (a Catalan proper noun in a Spanish text).

Here is a description of the space's goals, from Valencia's official website:
CASAL D'ESPLAI EL SALER

GOLA DE PUCHOL, S/N (EL SALER)
46012 VALENCIA
Teléfono: 96.161.09.36
El Casal d'Esplai Del Saler es un servicio de la Concejalía de Juventud del Ayuntamiento de Valencia, cuyas instalaciones se hallan en el parque natural de l'Albufera, en la zona mejor protegida y biológicamente mas completa.

Sus fines:
a)Servir de escenario a las actividades programadas por la concejalía de juventud: cursillos de animación socio cultural, colonias de vacaciones, intercambios juveniles...
b)Facilitar a asociaciones, colectivos juveniles y asociaciones prestadoras de servicios a la juventud el desarrollo de: cursillos reuniones, congresos... el tipo de actividades se realizan durante los fines de semana y periodos vacacionales.
c)Desarrollar las actividades de educación ambiental que promueve la propia concejalía de juventud "Escola de Natura la Malladeta", coincidiendo las fechas con el calendario escolar.

This is clearly for youth. In my experience, a "casal d'esplai" is always for youth--especially kids and pre-teens.

Note also that "Casal d'Esplai" is left without translation on the following page:
http://www.visitvalencia.com/en/valencia-beaches/la-devesa-b...

Other options are Casal d'Esplai youth centre (thus adding description but leaving the title, which is important if the guests want to find out more about the site where the event will take place) or simply youth centre.

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Note added at 4 hrs (2010-08-20 19:05:42 GMT)
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Also, on Whopdesign's webpage, the event location is written as "Casal d'Esplai":

"Whopdesign will be held from the 15th to 25th September, both days inclusive, at the Casal d’Espai de EL SALER (Valencia, Spain) a Concejalía de Juventud de Valencia centre."
http://www.whopdesign.org/p/about-en.html

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Note added at 4 hrs (2010-08-20 19:07:33 GMT)
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If you want to go along with their original translation, I might say "the Casal d'Esplai centre".

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Note added at 5 hrs (2010-08-20 19:16:06 GMT)
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Then I would say Casal d'Esplai (youth) centre.