Belgrade Fortress
I am not a fortress fan but you generally get nice views of the city as they are located on top of hills. It is also the case for Belgrade Fortress so if not for the fortress itself, just go there for the view.
Gardoš Tower
The tower was opened in 1896 to celebrate a thousand years of Hungarian settlement in the Pannonian plain. Today it is known after Janos Hunyadi who died in the fortress four and a half centuries before the tower was built. You will love the panoramic view a lot so make sure to pay a visit.
Clock Tower
It is situated in the entrance of the fortress so you cannot miss it. You can go up by paying a small fee but bear in mind the narrow steps you need to take!
St. Sava Temple
Church of St. Sava is one of the largest Orthodox churches in the world. It is under construction so there is not so much to see on the inside but worths a visit even only from outside.
Chapel of Saint Petka
The church is located in the upper part of the fortress, very close to the Rose Church. It was built on top of a spring, of which waters are believed to be beneficial to women.
St. Michael’s Cathedral
This is a very nice church located in the very heart of Belgrade, and very close to the fortress so make sure to check out its architecture and beautiful paintings.
St. Mark’s Church
St. Mark’s Church is located in the Tašmajdan park in Belgrade near the Parliament of Serbia. It is one of the largest churches in the country so you will definitely not miss it when you walk around the parliament.
Ružica Church
Ružica Church is located in the Kalemegdan Fortress. It was heavily damaged during the First World War, and was renovated in 1925.
Princess Ljubica’s Residence
Princess Ljubica’s Residence is very close to the fortress and aforementioned churches so you can pay a quick visit on your way/back to those places. Here you will find a collection of old furniture and art objects mainly from the 19th century. The house is decorated in such a beautiful way so you will definitely love it. 
House of the National Assembly of Serbia
This impressive building caught our attention while we were passing Nikola Pašić Square in one evening. You can chill at the nearby park while taking pictures of the nice building.
The NATO Bombing Ruins
The best-known ruins are Building of Ministry of Internal Affairs and Yugoslav Ministry of Defence, both are located in Kneza Miloša Street.
Nikola Tesla Museum
Nikola Tesla Museum was the only place I was dying to see but I must admit that I felt quite disappointed there. I really thought that I would have a chance to get to know about Tesla, which you can actually do at the museum, but I just expected to have more rather than just reading few general remarks on a TV screen. But interactive exhibition where models of Tesla’s inventions shown is quite interesting.
Trg Republike
Republic Square is the very centre of Belgrade. You can either make it start point of your trip or use it as a meeting point with your friends. You will find the National Museum, the National Theatre, Prince Michael monument and the Millennium Clock here.
Rivers
You should definitely spare some time to explore the riverside of Belgrade. Be it either Sava, the longest river in the Balkans or Danube, the large and mighty river of Europe. There are plenty of activities and parties during summer time but if you visit it during winter, you can at least enjoy the sunset at one of the cafes nearby.
Skadarlija
Skadarlija, also known as the bohemian quarter of Belgrade is definitely my favourite part of the city. It will make you feel as if you are walking streets of Montmartre! Skadarlija hosted many well-known but mostly poor poets and writers in the early 20th century. Alfred Hitchcock and Jimi Hendrix are among many of its famous visitors.Today you can find lovely restaurants , hotels, art galleries, antique and souvenir shops, and the Sebilj fountain here.
Room Escape Games
Belgrade room escape games are really fun and fully recommended. No matter if you visit the city during hot summer days or in winter times, it is worth sparing 1 or 2 hours for this activity. My personal favourite is Adrenaline Escape Rooms, with great staff and fun games!
Sea Green, Sea Blue 