El Mexi-Rock Cafe, 28411 Marguerite Parkway, Mission Viejo. Open 3 p.m. to 2 a.m. Monday through Saturday. (714) 364-6674. By day it's just another pastel-colored Mexican restaurant. But after 9 p.m., the rather small dance floor is generally packed with a youthful mix ranging from longhaired rockers to close-shaved hip-hoppers. The sound system is good, meaning that the only place for intimate conversation is out in the parking lot.