The antonyms for the word "most closed-up" can be "open," "unrestricted," "free," "uninhibited," "spacious," "airy," "expansive," and "accessible." These words imply a lack of barriers or hindrances, and a sense of ease and comfort. When a person or place is "open," it suggests a welcoming attitude towards new experiences or people. Similarly, "unrestricted" and "free" show a complete lack of limitations or restrictions. "Spacious," "airy," and "expansive" all indicate a sense of physical openness, with plenty of room to move around. Finally, "accessible" is used to describe something that is easy to reach or approach.