I'm with Ian - Brasilia fascinates me! I loved the brazen confidence that that Niemeyer showed in his design for Brazil's new capital in the 1960's.  I love the pomp and circumstance that you can nearly always find in the Eixo Monumental area.  I especially love sitting in the Catedral Metropolitana and just staring at the glass.   In my opinion, Brasilia can be well accomplished in a long day with a lot of walking.  The Praça dos Trés Poderes is the center park-like area with the key buildings surrounding the long plaza.   

If you just want to hit the most important spots, take a taxi (15 minutes without traffic), Uber or bus from the airport to the Biblioteca Nacional.  From there, you can walk along the south past the Museu Nacional, Cathedral Metropolitana and on down the hill to the Congresso Nacional building and then the various palacios and monuments.  At the giant flag, turn and head back up the hill to the Teatro Nacional.  At a minimum, try to visit the Cathedral, Congresso and the Panteão de Patria e da Liberdade Tancredo Neves with the eternal flame and amazing view back up toward the Biblioteca.   

The Praça is especially beautiful at night when the buildings are all lit up.  

You can join a walking tour or just get a map from your hotel or visitors center and walk the area on your own.  If you have time, don't miss the Santuario Dom Bosco - about 1km from the Biblioteca.