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As for Uklon taxi app, there's even an option checkbox "English speaking driver". Didn't use it, but may be helpful
Basically any hotel from Booking is as safe as any european hotel. If you are on budget and feel yourself as a tumbleweed person you'll like Dream Hostel , very cozy and cheap. Just see the photos. They even have transfer, but you can just Uber from airport, but for me as a local Uklon taxi app is more convenient. Taxi transportation cost like 3-5 euros inside Kyiv. As to where to go - you can start by just wandering around and seeing things, stick to Podil and Khreshchatyk for architecture. More trendy places are Poshtova ploshcha and Peyzazhna alley. Closer to October you can re-visit this thread to get a tips on what's going on in terms of fests and events. People around are friendly enough, younger ones know at least basic phrazes in English so you will have little trouble getting around. You can eat wherever Tripadvisor recommends, but I personally suggest you visit Пузата Хата for extremely cheap and tasty local cuisine. It's a canteen more than a resto, but the Podil one is cozy and like 6 mins walking from Dream Hostel, good spot to eat. Also there will be a great deal of local students from Kyiv Mohyla academy, so you can perfectly blend in and even make some friends.
Also I recommend to take a debit card with you. We have ATMs all over, and you can pay by card almost anywhere, so no point in carrying around much of cash.
As for Chernobyl: do you need a tip on tourist operator for the trip or this is already settled?