As in coffee, juice and tea. Best cafe is Entracte Cafe on Jomhuri Ave. Owned by award-winning Iranian actor Leila Hatami, which also happens to be very nice. On Fridays they serve up a really nice brunch between 11am and 2pm. It includes sausages, normal bread and brie! Fantastic. Also amazing is the traditional Iranian tea.
If you want trendy places try Gandhi Shopping center or one of the shopping centers a bit north of Vanak Square.
Kebabs are everywhere. Pizza is crap. If you want something else you can go for a very good curry and decent sushi at Monsoon in Gandhi shopping center on Gandhi Avenue. A good and very cheap traditional restaurant is located 200 m north of Valiasr Square. Another one but a bit more expensive but very good service is the one located on Ferdosi Square.
All hostels and budget hotels are around Amir Kabir street which is not a very nice area with no options for going out at night. Firouzeh hotel has the most amazin guy in the reception he makes it worth staying there and in that area. Alternatively try Hotel Naderi just 10 mins from there on Jomhuri Avenue. A cheap hotel further north would be nice but there are none.
Really not that much interesting to see in Tehran and the city itself is really quite ugly. The parks are nice though, I recommend Park Mellat north of Vanak Square or Park-e Shar if you are staying in a hostel on Amir Kabir and want to escape the traffic.
Take savarris (shared taxis) whenever you can. It is cheaper and you will meet more locals as well. They operate from square to square. Going from Imam Khomeini square all the way north to Tajrish square is 7.000 rials. The same distance by normal taxi is about 60.000 rials. The metro works good but has a rather limited network. If you are taking a bus somewhere try to take it from Arjentine bus station instead of the one in the south as it is in a nicer area. For the one in the south you can just take a metro so it is easy to get to.