"Girls often sing in cafés."
Translation:Knabinoj ofte kantas en kafejoj.
As a general rule there is never accusative case after prepositions, unless you're denoting motion, which you aren't here, as the girls are singing in the cafe, not into the cafes.
Mi iras en la kafejo = I am walking in the cafe
Mi iras en la kafejon = I am walking into the cafe