We do not own a car and manage fine.

Some holidays we hire a car, for example when camping in Guernsey, but mostly we use trains and buses. Examples of places where this is easy:

Austria - where you got a free bus pass on registering with the appartment owner
Verona - use buses
Venice - use water buses
French alps - use trains
Switzerland - local rail pass, so used trains
North wales - bus and train railrover (which included the Festiniog railway)
Lake district - use buses

etc

The crucial thing to do is to plan your holiday around public transport, so stay somewhere where this is possible. Do your research on the net first and find places which are local transport hubs. Or accept you may have to do some walking, which is of course good for you!

If you must stay in a remote villa only accessible by plane and car, then you have to accept your carbon foorprint will be poor.

Adam