"I offer you a drink."
Translation:Je vous offre à boire.
Not all words are directly translated because the two languages are not built the same way. In other words, you should not expect that every time you get "to" it will be "a" in French nor vice-versa.
"Je vous offre UNE boisson" is another way to say "je vous offre à boire". Just remember that "boisson" is feminine.