Mosjøen is located in Helgeland, and is actually exactly in the middle of Norway, between Lindesnes and Nordkapp. The town has a nice center with historic wooden houses, with mountains that rise majestically on the other side of the river. There are plenty of experiences to do in Mosjøen, also in the fall!
Øyfjellet (818 mas) is the mighty mountain that rises above Mosjøen center. The mountain is a popular hiking area but has several exciting activities. Øyfjell cave is one of them, a 500-meter long limestone cave. You can get a guided tour with Naturlige Helgeland, and can then borrow equipment such as overalls, gloves, helmets, and lanterns. There are a few crawling areas in the cave, so for people with claustrophobia, it can be challenging.
Fortunately, there are also other activities for both people with and without claustrophobia. Helgelandstrappen is one of them, a Sherpa staircase with around 3000 steps up the mountain. If you want a more challenging route to the top, you can try Via Ferrata, a climbing trail that takes around five hours from start to end. From Helgelandstrappen (which is also the way down Via Ferrata) is Northern Norway's longest zipline, at 700 meters. The zip-line goes from Øyfjellet, across the fjord and ends in the center, right by Sjøgata. You will get a stunning view on the way down.