Tehran doesn't really have much of a nightlife, especially not after midnight.
But there are some great cafes and restaurants where you can sit and enjoy while drinking a coffee or grabbing something to eat.
One great cafe is the Haftad-o Hasht where you can get coffee, tea and snacks. Another cafe is Naderi where you will see artists sitting and thinking about their next work. A 75-year-old cafe Gol-e Reziaeh is a place where you can get really nice 'home-made' khoresht.