Concierge.com's insider take:
When the weather gets hot in July and August, rent a car and escape to this Oslofjords island for sunning and swimming with the locals. Once inhabited by monks, Hovedøya is home to the ruins of a 12th-century Cistercian monastery, which burned down in 1532. In summertime, you might catch a medieval music performance beside the ruins. Be sure to take a leisurely walk inland to see the abundance of rare plants and flowers in this protected area; the flora includes mountain clover, wind flower, and blackthorn and barberry bushes.