• das Freilichtkino – “open air cinema”
• das Freibad – “open air/outdoor swimming pool”
• die Freifläche – “open space” or “undeveloped land”
• das Freilichtkonzert – “open air concert”
Frei is also a cognate of the English word “free.” Something that is frei is available at no cost (one can be more more specific and say that something is kostenfrei). In a related semantic field, there is the meaning of frei that corresponds to unabhängig –“independent” – which is a favorite word of mine as the Unabhängigkeitserklärung – The Declaration of Independence – was signed in my home town, Philadelphia.
Finally, one of the phrases I learned on my first trip to Germany in 1984 also uses frei: Ist dieser Platz frei? which means “Is this seat taken?” In this context frei means “unoccupied” or “not in use” or perhaps even “spare” if you intend to pick up the seat in question and take it to another location. I was delighted DWDS included this example sentence for the “available” or “spare” meaning of frei – Sie liest in jeder freien Minute, “She reads in every spare moment” – because it is so apt as far as I am concerned!