Bonn is nice with many good museums (Haus der Geschichte, Bundeskunsthalle etc.). In the region Schloss Augustusburg Brühl is UNESCO world heritage and worth a visit. At the Markt you can get good ice-cream (Eismanufaktur Solo Qui ).
Dortmund ("Verbotene Stadt") is more difficult. They have a modern art museum (Museum Ostwall im Dortmunder U) and the German Soccer museum. Otherwise it depends what you like.