One of Germany's largest technology museums, the Technik Museum Speyer sprawls across a massive indoor and outdoor complex. The star attraction is a Lufthansa Boeing 747 mounted on pylons — you can climb inside and walk through the entire aircraft, then ride a slide back down from the upper deck. There is also a Buran space shuttle, a submarine you can explore, vintage locomotives, and halls full of classic cars, motorcycles, and fire engines.
Plan at least half a day here; there is far more than you expect. The outdoor area alone could take an hour. An IMAX cinema on site shows nature and space documentaries. Combined tickets with the sister museum in Sinsheim (which has a Concorde) are available if you are doing both.