Depends on what you want to do. Be frugal or use time efficiently. I hate trains in Ukraine. I wouldn't care if they are free. You can fly from Odessa to Donetsk and take a taxi for about 40 dollars round trip. I would have your lady arrange the taxi or I can give you the number of a good taxi driver in Mariupol. She could meet you with Driver in Dontesk. a little over 2 hours from Odessa and you would be in Mariupol. You can book ahead for 75 bucks for the flight. No you don't need a visa for Ukraine.