Versailles alone need a good part of a day. Requires take a train to the suburb and it is BIG.
If enter British Museum or Louvre, each need about half day. Out side there is not much to see.
The Sistine Chapel, need get up early and stay in line for 2-3 hours. In side need about 1.5 hour; After that, there is another long line to enter St. Peter's.
Venece is not a easy place to get to. One day stay seems short.
Regular people, without training, can walk about 5 miles a day. Look at map, measure your walk see you can handle the length of the walk.