The two obvious day trips from Bodø are Saltstraumen and Lofoten, at opposite ends of the scale: one is a 30-minute drive to a natural phenomenon, the other is a full day (or longer) to an archipelago that deserves a week.
Saltstraumen (30 km southeast) is the world's strongest tidal current and one of the most dramatic natural sights in Norway. Drive or take the local bus along the Rv17; check tidal times before you go. See the separate Saltstraumen entry under Things to Do.
Lofoten is technically a day trip if you take the first express catamaran and return on the last, but it is cramped. The ferry from Bodø to Svolvær takes 3.5 hours each way; you get perhaps four hours in the archipelago, which is enough to understand why you should come back and stay longer. If your schedule allows only a day, consider an organised boat trip instead — several operators run day excursions from Bodø harbour that include some of the Lofoten coastline without requiring you to rush the ferry schedule.
Svartisen glacier is further afield (about 150 km south) but accessible by ferry across Holandsfjorden. Nordland Turselskap runs guided glacier walks from the visitor centre. Combine it with a leg of the scenic Kystriksveien coastal route if you have a car.
