At some point while learning German, it may have dawned on you that there isn't exactly a German equivalent for the adjective "fun." There is, of course, the noun der Spaß, which is used to describe how someone can "have fun" (Spaß haben) or something can "be fun" (Spaß machen):

Ich glaube, auch die Erwachsenen haben Spaß an dem Film.

I think adults also have fun with this film.

Caption 11, Michael Mittermeier: Hexe Lilli

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Und ich hab auch gelernt, dass es mir Spaß macht, vor der Kamera zu stehen,

And I also learned that standing in front of the camera is fun for me,

Caption 20, Anja Polzer: Interview

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The adjective spaßig does exist, but is more specific, meaning that something is either "merry" or "celebratory," or "jocular" or "playful." Es hat Spaß gemacht is therefore not really the same thing as Es war spaßig. 

Und spaßig ging es auch in der Festhalle weiter.

And it continued merrily in the Festhalle too.

Caption 24, Rheinmain im Blick: Live-Entertainment-Award in Frankfurt

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In terms of adjectives, however, there are a few other contenders. The adjective lustig can mean either "funny" or "fun" depending on the context.

Es ist einfach lustig und immer wieder da zwischendurch kommt trotzdem wieder ein Fund.

It's just fun, and time and time again in between, another find is still made.

Caption 64, Ausgrabungen: Auf den Spuren der Dinosaurier

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The adjectives amüsant and unterhaltsam also play a role here, as they describe something or someone being fun in the sense of being entertaining. 

Er ist sicher wahnsinnig locker und unheimlich amüsant.

I'm sure he's incredibly easygoing and incredibly entertaining.

Caption 28, Weihnachtsmann gesucht: Bist du verliebt?

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So unterhaltsam kann Lernen sein!

Learning can be so entertaining!

Caption 5, Theaterstück über gesunde Ernährung: Henrietta in Fructonia

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Für die knappe Stunde Flug ist die Außenansicht unterhaltsam genug.

For just under an hour's flight, the view outside is entertaining enough. 

Caption 10, Galileo Zug vs. Flugzeug: Von München nach Berlin

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Further Learning
On Yabla German, you can find many examples of how Germans cleverly use Spaß machen and Spaß haben to describe various fun occurrences and events. What has been fun for you in the last months? How would you tell someone about it in German?