RE: Need harder
Well, you could incorporate some pilates into your workout for more core work. I started pilates in January & I am truly amazed at the difference in my abs & transverse abdominals, not to mention the lower back & posture muscles. It has also made a world of difference in the rest of my routine. For example, somehow when I do pullups I can actually feel my core "levitating" me upwards--I have no idea how this works but it's really something.
Other than joining a gym (which would be my recommendation--I have fabulous back workouts at the gym b/c there are so many options), you could buy a pull up bar from your local sporting goods store. There are several different mounting options (pressure bars, brackets, etc.), but I used to have a bracket mounted bar & made great use of it. It is probably the most effective back exercise in the world.