The tourist office can suggest walks in the Syv Søstre (Seven Sisters) range, reached most conveniently via the Rv17 at Breimo or Sørra, about 4km south of town. From there you'll need to walk a couple of kilometres to the trailhead at the mountains' base. You can also get a taxi to the base. Trails are blazed with red dots and the tourist office can provide simple maps and basic route descriptions, but pack Alstahaug, a reliable map at 1:50,000. As a security precaution (the weather can dramatically change with little warning in these parts), let the tourist office know when and where you're going.
Hardy hikers can reach all seven summits (ranging from 910m to 1072m) in a day. Every several years there's a competition that takes in all the peaks – the record is three hours, 54 minutes, although most mortals should count on 15 to 20 hours as a minimum. The climb to Botnkrona (1072m), the highest of the peaks, takes most hikers of reasonable fitness three to four hours one way, while count on two to three hours to climb each of the other peaks (also one way).
Sign your name in the book at each summit, fill in a control card and leave it at the tourist office – in return, you'll receive a diploma in the mail.
Another possibility is the ascent of Dønnmannen (858m), an 8km-return hike that is rated as challenging and is for serious hikers only. The trails start at Hagen, on the island across the water from Sandnessjøen to the west. The tourist office in Sandnessjøen has route descriptions and can provide ferry timetables.