Rainforests and coral reefs . . .
Plants have been show to respond to direct stimuli, as have coral, but not necessarily to think, although it remains a possibility.
Coral, though, is much less populated and living in a 2D world. You can walk into a forest, but once you reach the coral, you're not going any deeper without a lot of help (picks, dynamite, a saw...). A healthy lump of coral is animal and may look like a brain, but I'm pretty sure most of it (the interior) is lifeless calcium mineral deposit, like a clam shell. A healthy forest, on the other hand, is entirely living, but still an inoffensive plant and harder to read.