Malls are dying. The thriving ones are spending millions to reinvent themselves.

Abdul Mannan was perched on a stool, scrolling through his phone, on a recent afternoon at Lakeforest Mall in suburban Washington. He was two hours into his shift at a jewelry kiosk and had yet to exchange words with a single customer.

There was a time, he says, when business was brisk, and the Gaithersburg, Md., shopping center teemed with shoppers. But that was before the J.C. Penney at the end of the corridor closed this summer. The Lord & Taylor went dark a few months later, and soon Sears will be gone, too...