A smartphone and translator app - To help decipher the Cyrillic alphabet used in Belarus. Invaluable in avoiding those embarrassing restaurant-menu situations!
A camera - How else are you going to capture pics of all the monumental Stalinist architecture to be found in Minsk, or the bison in the Belavezhskaya Pushcha National Park?
An adapter plug and transformer - Belarus runs on 220V. Appliances that take 110V will require a step-down transformer, and you’ll need a European 2-pin adapter plug.
An umbrella - Did we mention that Minsk weather is unpredictable? Expect occasional showers, even in summer.
Copies of your passport and credit card information - Obtaining a replacement passport here can be a headache, so make sure you bring copies. Hard copies, or scans backed up on a USB key - either will make life easier in the event of loss or theft.