Deutsches Museum Bonn is the Rhineland outpost of Munich's great science museum, but it is not trying to be a miniature copy. Its focus is modern German research after 1945: Nobel-prize work, medical technology, physics, communications and the sort of inventions that changed everyday life without becoming postcard landmarks.
It is a good counterweight to Bonn's Beethoven and government-history sights, especially if you are travelling with curious teenagers or need a rainy-day stop. The scale is manageable, the exhibits are more hands-on than hushed, and the location in the south of the city pairs well with the museum mile.