They were to （ have gone to） Sanya for an outing but bad wea

were to have gone to = would have gone to,是虚拟语气的用法,意为他们本来是要去三亚的（但实际上由于天气不好的原因去不成）。整句的意思就是：他们本是要去三亚的,但由于天气不好而被迫推迟。