A half-day trip to all the major cities in Europe without leaving Ontario, Canada
"all the major cities"
Ouch.
-- Germany, Spain, and a bunch more
On a similar note, you also could visit Moscow, Paris, Dublin, London, Vienna, Berlin, Warsaw, Florence, Athens, Edinburg, Geneva, Italy, Liverpool, Naples and Newcastle without leaving the state of Texas.
This is not to be considered a recommendation.
If you go to Paris, TX, you could take a ride on the Paris Metro.