Novi Sad is most known for the Petrovaradin Fortress, which is just on the other side of the Danube river from the center (maybe a 40 minute walk, partly uphill). You can enjoy walking up to the top, the view of the city and Danube river and having a drink at the restaurant. You can also visit the City Museum of Novi Sad at the top. Entrance costs 2 to 5 EUR, depending on which part you would like to see. More information can be found here: https://museumns.rs/en/
The Name of Mary Church in "Trg Slobode" (Liberty Square) in the heart of the city is a catholic church built in the neo-gothic style and can't be missed. The locals refer to it as "katedrala" (cathedral), even though Novi Sad belongs to the Roman Catholic Diocese of Subotica, whose cathedral is actually located in Subotica.
Google Maps link: https://maps.app.goo.gl/UtZybWfCBzam2UFdA
My family loves the fish soup at the restaurant on the Danube called Čarda Aqua Doria. The view is beautiful and the fish dishes are tasty, especially the fish soup, seasoned with lots of paprika and usually eaten spicy with noodles and cheese. A must try if you like fish! They also have many other local dishes, but fish is what they do best, in my family's opinion.
Google Maps link: https://g.co/kgs/naVHSP9
Trying Burek is a must when coming to Serbia and you can find it on EVERY corner. It is a family of savoury pastries or pies that is made with thin, flaky dough filled with cheese, meat, spinach or potatoes. It is most often eaten for breakfast (usually the cheese version) with a drink called Jogurt. The best burek in Novi Sad (according to us) is Burek Plus: https://g.co/kgs/cRXwTUp
Sweets/Cakes (only to go) - Slatki Dan: https://g.co/kgs/PxYbH9F
Regional Food - Plava Frajla: https://g.co/kgs/AZJRfE2
Bosnian "Cevapcici" - Sarajevski Ćevapi kod Dakca: https://g.co/kgs/eENyyZM
Ice Cream - Crna Ovca: https://maps.app.goo.gl/RxVw96cc75VbtEW27
Index Sendvič (Flo loves this one): https://g.co/kgs/iZEHZ8w