Kastelholm Slott is a easy daytrip from Marienham.
Go there by bus nr. 4, 35 minutes, not a lot of connections, the last bus back is at 6.39 pm wekdays, 5.39 pm saturdays and 5 pm sundays, you will have to walk 500 m from the first possible bus stop and 400 m from the second possible bus stop. Both stops have signs, the second also signs for info etc. You could also go by rented bicycle, it's only 23 km each way.
Right next to the castle is the Jan Karlsgården open-air Museum, quite interresting as well.
Eckero market at the market place in Storby is a popular meeting place every Wednesday during summer. Starting at Midsummer till late August the market provides handcrafts, local products, fish, bread, vegetables, art objects, souvenirs and everything else a good market should provide. Refresments and activities to children (horseback riding). Open 10am-3pm.