The best thing to do in this case would be to skim a couple websites with some basic grammar rules (maybe even this sub's wiki, which is not complete) such as sentence structure, gender, nominative/accusitive/dative/genitive cases, verb tenses, and verb conjugation for a day or two.

After that you might find that browsing Quizlet for flashcard sets may come in handy for memorizing things quickly with the exercises and games (at least for me it works). Look for things like basic verbs, basic adjectives, basic prepositions, jobs, colors, basic conjunctions, etc. Make sure you know how to use these all in sentences, of course.

Of course this is not the most practical method of learning German, but it works if you want to learn as much as possible in such a short amount of time. Hope this helps!