I think you're asking the question the wrong way around.
First research what you want to see and where those targets are. Then worry about how to get there.
Case in point: A few years ago we lucked out by being in Belgium on the day of "patrimoine" - heritage day when all of Belgium opens its doors, even if only on that one day, and lets the public look in. Our host (we stayed with private, through a hospitality exchange organization where we have long belonged) drove us to a lot of small towns and distant castle ruins and such destinations (including a snail farm, the culinary kind of snail...) that we would not have been able to reach by train or bus, even if (and that's a big IF) we had known about them.
In hindsight, had we known about all those and had we been on our own, we would no doubt have rented a car.
So dig deep into what's there to see, outside of the obvious towns that are easily reached by train, and if there is a list of targets that seem worthwhile, then figure out if there is a bus or if renting a car is the smart way of doing it.