Some foreign buyers see owning a Manhattan apartment as just another custom neon yellow ostrich skin Birkin bag—or Swiss bank account. Meanwhile, most New Yorkers would give a kidney for an extra bedroom, and some legislators are trying to institute a tax on high-end apartments whose owners live elsewhere. Either way, ​as the New York Times reported this weekend, acres of high-end real estate on Billionaire's Row and other tony Midtown addresses lie empty. But we have a few ideas to put all that vacant space to work: