A combination of ferry between the islands and a bicycle on the islands themselves is the most popular option. There are well developed facilities for bicycles including dedicated cycle paths along many roads and ferries exclusively for bicycles.
There is a good road network and the island can be crossed by car in less than an hour between furthest points. Traffic is very light all over the island including in Mariehamn. Renting a car is easy in Mariehamn. Fuel is widely available around the island and is similar in price to say mainland Sweden.
The trip to Åland through the eastern archipelago coming from Finland is something you will never forget. Choose a route through either the southern or northern archipelago. Bookings can be made for trips to and from an intermediate port. Trips from one destination port to another can only be made if you spend a night on one of the small islands. The archipelago ferries is served by Ålandstrafiken (http://www.alandstrafiken...).
The nearby island of Vårdo can be reached by free cable ferry.