#How does tel/telle/tels/telles work?
vale pendant
it is an adjective, yes. The "de" in that sentence is the indefinite article "des", which becomes "de" when it's right before an adjective in formal speech.
sullen geyser
it is an adjective, yes. The "de" in that sentence is the indefinite article "de...
So it's supposed to be "que celle des telles activités offrent" and is de in formal speech. "I want a life more calm than that that (some) such activities offer"?
vale pendant
“Que celle que des telles activités offrent”, yes. I imagine omitting the “que” was a typo, but just thought I’d highlight it just in case.
“… than the one that such activities offer”