One of your first stops should be Fredrikstad Tourist Office. Here, you'll be able to orient yourself and pick up some great ideas before you begin exploring.
When you've finished at this tourist information center in Gamlebyen, why not check out Fredrikstad Museum, Kongsten Fort, and Gamlebyen Goat & Poultry Farm? They're not far from Fredrikstad Tourist Office and also a favorite with many travelers.