We have 2 the most beautiful and famous castle in Belarus, this one is located in Nesvizh and was owned by the Radziwill magnate family for many many years. I visited it at the beginning of September, and I can surely say that early autumn is the best time for going there! There is a very beautiful park around the castle and it's more charming in this period - when centuries-old trees have different colors.
Minsk City Hall. It was rebuilt several times during 16-19th centuries, then in the fifties of the 19th it was completely demolished and only at the beginning of the 21th it was rebuilt again. I thought that it's quite old and only having searched some information for this blog I realized that it's not :)
"Belarusian Maldives" or Krasnoselsky chalkpit - the most beautiful and difficult to access place in Belarus I've even been to! If you plan to come there without a car be ready to have a looooong walk on foot: my sister and I walked around 10 km from the closest train station Ros' to get there. But it was worse it! You'll fall in love with these views and will want to come back again and again! :)