Here are only ideas listed which have available informations in English. It might be interesting for you to have a look at the extensive German collection

in Freiburg
Sightseeing flights and much more: helicopter, light aircraft, aerobatic aircraft, Biplane, tandem parachute jump, ballooning, airfield guided tour and Tower visit
City Museums
The 5 Museums at one Homepage
Disused Mine Teufelsgrund
at Münstertal
April to October, opening days and times at the homepage