In my experience they learn to open gates. They certainly aren't trained to do this, but learn from watching people or each other. They will also open a gate

In my experience they learn to open gates. They certainly aren't trained to do this, but learn from watching people or each other. They will also open a gate to let another horse out of their stall which I would count as some form of planning. Beyond that I can't think of anything in all the years around them. They can manage to be surprised by the same things every single day.