Renting a car to tour around Andorra is a very smart option because your vacation won't be dependent on regular public transport routes, schedules, and other people. You won't need an international identity card or an international driving license to drive a car in Andorra, as long as the purpose of the visit is listed as "tourism."
There are a lot of car rental offices in Andorra. It's wise to arrange a car before you arrive in the country. You can check options and availability at https://www.holidaysandorra.org/car-rental-in-andorra .