Want to brush up on grammar over holidays? Writing exercises, going over things you already studied will further strengthen your grammar and vocabulary. Practice Makes Perfect Spanish Verb Tenses is a book I highly recommend. This is not for beginning students, it is more of a review grammar exercise book. While I really don’t believe in translation exercises, because the point is to speak without relying on literal translation, each language having its own grammar rules and quirks, in this case, you can make an exception using this book.
I recommend doing an exercise a day. Remember, it’s quality, not quantity. Do the exercise, review it and move on to the next the following day.