The best grammar site I know of is "Basic Spanish for the Virtual Student." Below I have included a more detailed explanation for classroom activities related to this link.

Introduction
"Basic Spanish for the Virtual Student" presents grammar-based activities for the beginning level Spanish student. This site has a lot of potential for out of class activities and reviewing. One of its greatest assets is the index, where a simple click on a particular grammar point leads to several hints and explanatory charts. For the more advanced student this site might be a handy review source.

The site is divided into eight pages of organized grammar topics, each page covering 1-10 grammar points. It can be helpful in preparing students for exams and for writing compositions. I would also recommend it to students who, have not come to grips with specific concepts when they were taught in class. Though some of the sections are more complete than others, they all give at least a minimal outline of the grammar concept. Since the pages are linked together, the student can easily choose to jump forward to more difficult grammar information, or backward to the more basic concepts whenever necessary.