Online exercises/activities:

For practice with verb forms (present, preterit, imperfect, subjunctive, commands), go to Verb Form Practice and click on VERB DRILL.  These exercises are intended for formation-practice and -learning only...but as always you should think about meaning and usage as you "fill in the gaps."  Thanks to Melinda Johansson for creating and offering these exercises.

For other items, click on the topic you wish:

Grammar exercises (this one has ALL verb tenses)
Verb conjugation (this one is "conjugation-only)
Adjective clauses
Noun clauses of doubt
Adverbial clauses
Tú commands
Participles
Spanish Grammar Tutorial