Assuming you can, mainly walk around Central Kiev; it’s a fairly easy city to navigate, and has lots of interesting buildings.
The metro works on tokens; I think you only use the token when entering, but double check that. It’s very cheap, and station Arsenalna is the deepest in either Europe or the world. There’s also a cosmonaut themed station with pictures of the men and women by the platforms. The signage is all in Ukrainian, but when you know how the letters are pronounced, place names are relatively easy to decipher.
St Andrew’s church is worth a look, and what I think was the monastery, but the name escapes me. The photo tag says Perchersk Lavra.
Pripyat and Chernobyl and astonishing; thought provoking and quite unlike anywhere else you’ll have been. Wear layers; even if it’s chilly, you’ll walk a lot and get quite warm. Walk on tarmac where you can and try not to walk on moss. Try to separate from the group (but keeping them in sight) and just take in the sights and sounds, you’ll never forget it.
I went to Ukraine expecting an amazing time, and it exceeded my expectations.
Fingers crossed for you that everything goes to plan.