Most traditional way is making simit plane with very simple dough. Then, cover it up with sesame seed. Turkish people usually enjoy it Turkish tea.
However, there are many kinds of simit in Turkey:
In Afyon, they put poppy seed on top of simit instead of sesame seed.
In Turhal, there is nothing on the simit. It is served plain.
In Beypazari, the simit is made with chickpea yeast.
My favorite is the Armenian simit called “çorag” a.k.a "Kandil Simidi".
Here is a great article that tells more about simit culture in Turkey.