Learn Finnish: Katselumaraton osoittautui hauskaksi, mikä yllätti minut. - The binge-watching session turned out to be fun, which surprised me.

The ending -ksi is the translative case marker. The verb osoittautua (to turn out to be) requires it because it describes an outcome or a state that something resolves into. Even though in English we say "turned out to be fun", in Finnish the word for "fun" must take this outcome case, becoming hauskaksi.

Could I use joka instead of mikä to mean "which"?

Not in this sentence. The relative pronoun joka refers back to an immediately preceding noun. Because "which" here refers to the entire previous clause (the whole fact that the marathon turned out to be fun), you must use mikä.

Why is the object minut rather than the partitive minua?

The verb yllättää (to surprise) takes a total object here because the action is completed. For personal pronouns like minä, the total object takes a special accusative form ending in -t, resulting in minut.

I know katsoa means to watch, but where does the -elu part in katselumaraton come from?

Katselu is a noun meaning "viewing" or "watching". It is derived from the continuous verb katsella (to spend time watching). Combined with maraton, it creates a compound word that perfectly captures the modern concept of a binge-watching session.