I think that I accepted it in some sentences, but the more I think about it, the more I think that I shouldn't have. They are just different verbs. "to relax" is "relaksować się". So "Relaksujemy się we włoskim hotelu."
Under what circumstances would an English speaker say this specific sentence though? This translation seems too literal. I'd translate this as relaxing/vacationing/taking a break, because that's the gist of the Polish sentence. (Also, you can rest "at" as well as "in" a hotel. But your system only accepts "in".)
I guess you're right, it's not exactly natural in English, but all your translations are different verbs. I'd say that this is one of those sentences in which you can't be natural in both languages, given how Duolingo works.