Learn Hardcore German: Fitness macht Spaß.

In German, Fitness here is treated as a mass noun referring to the general concept or activity. Many words for sports or activities (e.g. Sport, Jogging, Schwimmen) can appear without an article when you talk about them in a general sense. So Fitness doesn’t need ein or die in Fitness macht Spaß.

What case is Spaß in, and why is there no article before it?

Spaß is the direct object of macht and thus in the accusative case.
In the common idiom Spaß machen (“to be fun”), Spaß often stands as a bare noun without an article when used generically. You could say Der Spaß macht mir …, but it’s normal to drop der here.

How is Fitness macht Spaß pronounced?

Phonetic approximation:
fɪt·nɛs maxt ʃpaːs

Stress on the first syllable of Fitness and on Spaß.
The -ss in Fitness sounds like a short /s/.
The ß in Spaß is a “sharp” /s/ following a long vowel (the /aː/).

What does the expression Spaß machen literally and idiomatically mean?

Literally: Spaß = fun, machen = to make ⇒ “to make fun.”
Idiomatically: Spaß machen means “to be fun.”
So although word‑for‑word it looks like “Fitness makes fun,” the natural English is “Fitness is fun.”

Why is Spaß spelled with ß instead of ss?

German spelling rules use ß (Eszett) after long vowels and diphthongs. In Spaß, the vowel /aː/ is long, so it’s spelled ß. If the preceding vowel were short, you’d see ss instead.

What are the genders of Fitness and Spaß, and why don’t they appear here?

Spaß is masculine: der Spaß (no regular plural when abstract).
Fitness is feminine: die Fitness (usually uncountable, so no plural).
In Fitness macht Spaß both appear without their articles because they’re used in a general, idiomatic way.

How would I turn Fitness macht Spaß into a question?

You simply invert the verb and the subject:
Macht Fitness Spaß?
This literally asks “Does fitness make fun?”, i.e. “Is fitness fun?”

What’s the difference between etwas macht Spaß and Spaß haben?

etwas macht Spaß (“something makes fun”) = “something is fun.”
Spaß haben (“to have fun”) = “to enjoy oneself.”

Example:
Fitness macht Spaß, wenn du mit Freunden trainierst.
Ich habe Spaß, wenn ich mit Freunden trainiere.