Örö was long a military island, first containing a defensive fortress built under Russian rule and later serving as training grounds for the Finnish military. The island's barracks, heavy artillery battery (complete with guns) and fortifications on the northern tip were opened for exploration in 2015. The MS Ejskär makes the 1¼-hour trip between Örö and Kasnäs once or twice daily in summer and at weekends in September. Book ahead.
Because the island was closed for so many years, it's home to unique birds and butterflies, as well as rare plant species, such as pasque flower and sea kale. Birders will have a field day, as many migrating Arctic species stop here. The island is also a nesting place for the icterine warbler and the barred warbler (both can be seen in June).
There's a variety of accommodation, including a hostel (dorm beds/twins €35/110) in the old barracks, and a restaurant in the canteen.