There is always the obvious - Dealey Plaza. Within blocks of that you have the Old Red Courthouse and Reunion Tower (Dallas' phallic symbol). In the Arts District about a mile from downtown there is the new Opera House under construction; an interesting modern design. Frank Lloyd Wright's only Dallas building is the Dallas Theater Center. Sixteen miles west of Dallas in Arlington (my Home) is the new stadium for the Dallas Cowboys. For landscapes, or at least plantscapes, there is the Dallas Arboretum.
Enjoy your visit.