A national park in south Jutland just across the Kalo Vig from Aarhus, Mols Bjerge is one of the premiere outdoor adventure areas in proximity to the city. From the ruins of Kalo Castle to the viewpoint on Jernhatte over the sea, there is plenty to do while in the park. You can hike to most of the park highlights within half a day; just take the worn path from the parking lot that will take you through beech forest, hill and seaside views and down to the waterside for a swim. You may then follow the beach back to the parking area.
Note:Plenty of info fishing and swimming locations, as well as which of the Mols hills you should be taking on, can be found on the included website.