I recently quit my gym as I went to college and can no longer afford it. Now, I use Jillian Michaels DVDs, but that said, I really liked using the gym.
I did strength training using dumbbells. But I really liked using exercise balls during the exercises because they helped stabilize my core. As for cardio, I loved using the stairmaster. I don't know if that would be good for your back, though. I also like to run.
I do about 4 times a week with strength, 5 times a week with cardio. I mix and match to my heart's desire with how much of each depending on my schedule. For example, an hour of strength one day, an hour of cardio the next. The next day, I have only a half hour, so I mix it up with supersets and sprints.
I hope this helps.
Have fun working out! I now consider myself a gym rat and enjoy it and I used to hate it before. If I can like it, anyone can!