We are likely to use a car this year, but it's not primarily a Rome trip. Many people combine Villa d'Este and and Hadrian's Villa, by train, from Rome. But I think that would require a cab for Hadrian's. When we went to the garden (d'Este), we didn't have a car, but I forget if we walked from the train station. Many people recommend a full day for serious exploration of Hadrian's Villa, but I suspect half a day is enough - haven't been there yet. For effortless but no control of timing, consider Viator - I don't mean to sound contemptuous, we've used them in places like Lisbon or Athens, where there was a lot of distance to cover and sights to see.