Semnan's sights cluster around the old centre. Start at the Semnan Citadel Gate, then continue toward Semnan Bazaar and the Jameh Mosque. The distances are short, and the pleasure is in piecing together the remains of an old caravan city now surrounded by modern traffic.
This is not a place that overwhelms you with monuments. It rewards a slower eye: brick geometry, tile panels, old shopfronts, and the sudden view north to the Alborz when the air is clear. For cooler air and mountain scenery, continue west to Mahdishar.