Tribal tattoo designs have been very popular on the lower back. Don't freak out, tribal tats can be very simple designs. They don't have to be big and complex. Matter of fact you can take many standard tattoo designs and transform them into tribal. Birds or crosses for instance can have tribal-like versions which can work very well. Research your tribal design well, and learn the meaning that different designs may have.
Flowers tattoo are great for lower back designs. But instead of just having one flower, consider multiple flowers. These really get noticed and are much stronger than a single flower design. Some girls have even got mural flower designs that really pop out, and are visually beautiful. But if you want to start with one flower, then that's fine. Research the different flower designs and pick the one that fits you perfect!
Vine tattoos Here's one you may have not considered, but vine tattoos can look great as girl back tattoos. Leaves are usually attached to the vines, and the design can stretch all the way across the back. You can also incorporate different designs withing the vines, such as birds or fairies or flowers.