While city hotels aren’t usually on locals’ radars as beauty destinations, make room in your life for this one. With a membership, the Boston Harbor Hotel’s iconic archway becomes your gateway to a newly renovated fitness-and-spa heaven, full of state-of-the-art equipment, luxury treatments and a 60-foot saltwater lap pool. Greet each morning with a spread of fruit, juices, tea and newspapers, and find locker rooms stocked with robes, shaving cream and even workout clothes and earphones.