Serena Smith spent a summer weekend at an Airbnb on the Jersey shore, laying out by the pool with her laptop in hand, going out to dinner every night and frequenting a beach within walking distance of her rental home in Stone Harbor, N.J. The Crossmedia analyst, who lives in San Diego, wasn’t technically out of the office—she was on retreat with her coworkers at one of five summer houses the agency rented this summer.