Berlin
Berlin is the capital of Germany. The city has three airports "in a row" which you will be visiting all. Start at Shonefeld (EDDB) and fly north-west to Tempelhof (EDDI). The ground 
and the old terminal buildings from 1927 
of Tempelhof are somewhat characteristic. Fly north from EDDI to the city centre where you will see a lot of famous buildings, e.g. the Reichstag and Brandenburg Gate. Continue to north-west and land at Tegel (EDDT).
Comments:
You should do it as long as all airports are existing. The city government (Senat of Berlin) decided to close EDDI and EDDT soon after the planned upgrading of EDDB has been finalised.What??! EDDT will NOT be closed!!!!! Only THF (EDDI). But EDDT not!! Operations in EDDT will continue.
Nope, EDDT will be closed when Schoenefeld (Berlin-Brandenburg International thence) will become fully operational. It was in the Berling Newspaper media in 2005 (look for +tegel +schliessen in Google). I know of no world city like London, Paris, Chicago, Tokio that closes all airports just to have one. It's a sad, sad, story.
Well... and no other megapolis has two airports within the city borders, one even right in the city center.