Coral Reef, try for dinner around 7 p.m., if you want to be able to see Illuminations that night.
I'd imagine that Rainforest gets pretty busy at park closing. But you can take up to an hour or so to exit the park, so you might have good luck with a ADR for an hour after closing. As long as you have plans for all your table service credits, paying for one meal isn't a crazy idea. Rainforest is a fun place to try once.