Hello,
We will be traveling to Elba in early June 2020. We plan to take the ferry over from Piombino and would like to have a car while on the island. In addition, after our stay on Elba, we will be spending a day or two driving through Tuscany en route to Florence.

Which scenario is the best plan?
Option 1: Rent a car on the mainland, take it on the ferry to/from Elba and keep it until we drop off in Florence.
Option 2: Wait and rent the car when we get to Elba (Portoferraio), then take it on the ferry back to mainland and drop off in Florence (do they let the Elba rental cars leave the island?).
Option 3: Do two separate rentals - one on Elba, then a separate one when we get to the mainland for our drive up to Florence.

Thanks in advance for your help!

We have been to Elba about 10 times and we always rent a car before arriving on the island. It wouldn't make sense to carry all your luggage on the ferry as a foot passenger or lug it on the train to Piombino. Rent the car when you are ready to leave whatever city you fly into. It is very easy driving the car onto the ferry and then once you arrive in Portoferraio you can go straight to your destination.

Check that the rental can be taken on a ferry.

Yes rentals can be taken on a ferry. Every time I've gone to Elba I've driven a rental car on the ferry. Sometimes when you buy the ferry ticket you have to list the type of car, but you can just stick in a model similar to what you are renting - it's just for the size. They don't check it.

I was looking at it from the rental agency rules. For example, cars rented in France cannot be taken on a ferry to Corsica. I am not sure that cars rented on the Italian mainland can be taken to Sicily. Perhaps the rule varies according to the rental agency, and it would be safer for the OP to inquire.

Thanks so much! I think we'll rent in Piombino before we get on the ferry, then drop off in Florence after our drive through Tuscany! Can't wait!

How will you reach Piombino?

We'll be arriving in Piombino via train from Bologna

In this case, i woudl rent the car at Bologna and avoid the train changes at Florence and Pisa.

I basically agree, although I would probably rent the car in Florence, because I hate the drive from Bologna to Florence.