The youthful, artsy vibe here is courtesy of the University of Iowa campus. It spills across both sides of the Iowa River (which has good walks on the banks); to the east it mingles with the charming downtown. In summer, when the student-to-townie ratio evens out, the city mellows somewhat.
The school's writing programs are renowned, and Iowa City was named a Unesco City of Literature in 2008. For a sharp parody of the town and school, read Jane Smiley's Moo. If you have time, the University of Iowa has good art and natural-history museums.
Note that neighboring Coralville is everything that Iowa City is not: over-run with chains and strip malls.