According to wordreference.com, "arena" in Spanish would be «estadio» for sports activities, and "quadrilateral" would be «cuadrilátero»: http://www.wordreference.com/es/translation.asp?tranword=arena#
I think a boxing arena is usually a "square ring," cuadrilátero. But, I think "arena" can also have a sporting/competitive sense: "la arena politico" or "arena de deportes." Oddly, the one sport I think of that is definitively Spanish, and played out on sand - bullfighting - isn't, in my limited knowledge, usually called an "arena?" I stand to be corrected, but, the two bullrings I'm familiar with are both called the "(place name) Plaza de Toros." As we ran into earlier in this module, "campo," is a "field," and I've seen "campo de futbol" and other similar use for sport. I wonder what you call a beach volleyball pitch in Mexico? Certainly sandy and sporty!