Thanks for asking.
Hyperextensions are a good lower back exercise.
Deadlifts, both straight and bent-legged give the lower back a lot of exercise, so if you do them you don't need to add anything else. Make sure you get someone to show you how to do them properly and keep your lower back arched through the whole movement. For more details consult the misc.fitness FAQ which contains extensive descriptions of both sorts of deadlifts and lots more besides.