were they very intelligent or as thick as a plank of wood?
A part of it depends on what we define as intelligence. There are certain species of jellyfish, the box jellyfish, that have no brains and yet they can navigate obstacles in a tank and even learn where those obstacles are. They have a dozen microscopic eyes and we have never seen an animal with no brain and eyes before. We don't know how they interpret what they see without a brain. There may someday be evidence that a large brain isn't the only thing that determines intelligence. A part of it could be the information encoded on the DNA molecules which dictates what that animal does. I think some dinosaurs could have been pretty smart.
Recent studies show that birds display a remarkable intelligence even with a very small brain. Since birds are closely related to dinosaurs, it becomes really difficult to say that dinosaurs weren't smart because they had small brains. It is likely that much of their behavior was instinctual but it is hard to determine with limited fossil evidence. We have foot trackways where we can see some interactions and nest sites where wild guesses can be made.
Unfortunately brain tissue doesn't really fossilize, so we can't analyze their brains directly. But relative to their bodies, most dinosaurs appear to have pretty small brains. But that doesn't mean they were stupid -- there is evidence of several types of social behaviors that are strikingly similar to many animals we see today. So while dinosaurs definitely weren't at the level of dolphins or chimpanzees, they probably weren't pathetically stupid either.
Since no one was alive then, all we can do is speculate. But I imagine, like animals today, some were dumb as a bag of hammers and some were maybe as intelligent as some animals alive today.
I suspect they were intelligent and varied much like animals today.
sounds like they would be very intelligent if they were that big