Exercises:
http://www.englisch-hilfen.de/en/exercises/pronouns/relative_pronouns.htm
http://www.perfect-english-grammar.com/relative-clauses-exercise-1.html
http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?02
http://www.englisch-hilfen.de/en/exercises/confusing_words/what_which.htm
http://www.englishexercises.org/makeagame/viewgame.asp?id=3221