The Journées du Patrimoine (heritage days), being held this weekend, need not just mean long queues to the city's principal political and cultural institutions. The event, featuring over 300 sites across Paris, also offers many out of the ordinary locations where access is only given during this weekend.
You can find my suggestions on the Paris Weekends blog.
If you are elsewhere in Europe, don't forget that the heritage days event now extends to 50 countries across the continent. The full list of events per country (as well as the local dates) can be found here: http://www.coe.int/t/dg4/cultureheritage/heritage/ehd/National_Events/map_en.asp