That is one of the most beautiful areas in California. For a day-trip, drive the two-hour GEORGEOUS coastal drive through Big Sur down to San Simeon where you can take a tour of Hearst Castle. (I've only been to Hearst Castle once but have wanted to get back. Look it up online and see if it's something you'd be interested in - for me, it's not to be missed.) And then a little further is Cambria which is a totally cute little town with shops and restaurants. Would be a great spot to stroll through the town for some shopping and a nice dinner before you head back to Monterey.
If you don't want to drive the curvy roads along the coast in the dark, maybe head out earlier in the day, stopping in Cambria first for a stroll through the town and nice lunch in one of the restaurants. Then Hearst Castle tour then head back to Monterey late afternoon while it is still light. And if you time it right, you could be almost back to Monterey and find a perfect viewing site along the coast to stop and enjoy the sunset!