What was once a cell-phone-accessories warehouse will become one of the largest indoor climbing gyms in the country when it opens this week as the Cliffs at LIC (11-11 44th Dr., Long Island City; 718-729-7625). Welcoming monthly members as early as May 4 and walk-ins starting May 10 ($95 and $25, respectively), the rainbow-walled brainchild of mountaineer Mike Wolfert has 60-foot ceilings; 30,000 square feet of bouldering, top-roping, and lead-climbing space; and, for those who prefer less-vertical workouts, a sea of cardio and weight-training equipment. Unlike its kiddie-centric sister location in Westchester, birthday parties here are relegated to a sectioned-off area—making the space appealing to ascenders of all ages.