This is really funny! I've been searching about this for a while now. I think it should be a very frequent word that I can instantly recognize upon hearing but no words come to my mind. Let's say there is a park which is very crowded on the weekends and not so much on workdays. How would you describe the park in those days that not too many people go?
How would you fill the sentence "The park is [blank] today!"?
My problem with "less crowded" is that it's not really the actual word but rather we're trying to convey the meaning by using its antonym. It's like calling a well-lit place "not dark." It is true but there is already a direct word for that meaning. Upon searching I found some other words that could fill the blank like "secluded" or "solitude." But I feel like "secluded" is a place that is always empty of people. Not a place that is less crowded from time to time. You cannot say the park is secluded today! That sounds ridiculous to me! There are also words like "vacant" that seem like overkill. I guess there are no people (or maybe a handful of people) in a vacant place.
I have a unique word for this in my own native language and when somebody asked me for its equivalence in English, I really couldn't figure it out.