One of the most common stands you'll see is for dried sour plums and fruit roll-up, lavashak. There are all kinds of flavors including plum, berries, and sour cherry among others.
Other common snacks include fresh berries like shah toot, blackberries, and toot, mulberries. Plenty of goje sabz is also available. These little green plums are so sour they are eaten with salt to balance out the taste.
Jars of fresh gerdoo, walnuts, soaked in salt water and baghali, fava beans, are other favorites. You can add some vinegar and golpar, angelica seeds, to the baghali. And when the weather is cold, there is the nice option of delicious ash reshteh, noodle soup.
If you have a sweet tooth, pashmak, cotton candy, might be a nice option!
This final one isn't so much of a common street snack. I just came across this interesting chip flavor. As it turned out, kebab flavored chips are the Iranian version of BBQ.