What to see largely depends on how much time you can spend in-country. If you fly there you will almost certainly fly into the capital city, Minsk. Hotel prices are surprisingly high there. A modest hotel not in the city center will cost $90. A better hotel in the city center will cost more like $200, and prices go up from there. The metro system is good, and cheap. There are nice parks throughout the city. Very little of the city survived the war, so you won't see much of the 19th century architecture you might see in other cities of the FSU.
And speaking of war, Belarus has had way more than its share of war. If you're a history buff you might find that interesting. Napoleon invaded Russia through what is now Belarus. So did Hitler. And the armies of both dictators retreated that way too. If you have time and are interested in the war, you might pay a visit to the fortress at Brest, in Western Belarus near the Polish border, where in the first week of the invasion a few thousand Soviet soldiers held off 20,000 Germans for a time.
There are beautiful lakes in the North of Belarus, near Braslaw.
If you're in the North of the country, I would recommend seeing the city of Polotsk, which was founded in 864 and was an important city for several centuries afterward. St. Sophia Cathedral there was built I think in the 11th century. It's situated in a beautiful location on a hill above the river Dvina.
And last, but certainly not least, there are so many gorgeous women in Belarus. A fair number of them speak English too.