Hello Åse
Welcome to the forum!
Trinidad & Tobago were once the third largest producers of cocoa in the world, during the late 1800s. However, it has sadly declined and now I believe that total production is less than 1000 tons per year and most of that is produced in Trinidad.
Visitors can get a glimpse of how widespread cocoa production once was from the glorious orange-flowering Immortelle trees that were planted to provide shade to the cocoa.
To be perfectly honest with you, Åse, production on Tobago is now very small-scale and I don't know of any plantations that are open to the public. However, you will see lots of cocoa plants around the island. My best recommendation, however, is to take an off-road safari with Tobago Now or Alibaba as these tours get off the beaten track and pass through old plantation areas. In fact it is worth contacting these businesses to see if they can arrange something more specific for you.
Yes, it is easy to drive around Tobago but there are also small-bus tours. If it is your first visit, the BEST way by far is to take an island tour with one of the established operator - such as those mentioned above or Yes Tourism - to give you a "feel" for the island and a general overview of things, and then hire a car and explore at your own leisure.