Summer
This is a good time for travelling. 20 days won't be enough to visit all the interesting places. So you'd better choose one area and fully enjoy your choice.
Say you're a car enthusiast. In all major cities, there is car rental and good guides. It's better to choose itineraries by region. Western, Central, and Southern Ukraine are the most interesting ones. It doesn't matter whether it is visiting castles (Volhynia, Lviv, Bukovyna) or a trip to Askaniya Nova Reserve, or to Khortytsia — you are always welcome there, served delicious food, and shown around.
In Ukraine, there are many caves with beautiful galleries and underground lakes. To become a caver, you should go to Ternopil, Donetsk, or Crimea. Experienced instructors will teach you to listen to the sound of silence and appreciate peace. One of the longest caves in the world is called Optymistychna (Optimistic) and is located in Ukraine. Especially interesting are the cave towns of Chufut Kale and Manhup Kale in Crimea Peninsula. Under Odesa, there are old catacombs.
Jump at the chance to visit the Carpathian Mountains — beautiful waterfalls, breathtaking views, horse riding and cycling, trout fishing and watching animals in the wild. The most interesting place in the Carpathians is the old observatory on Mount Pip Ivan. Today, it is being restored. To watch the Milky Way from its walls is a dream of real travellers.