First of all welcome to this great app .... You can do squats and jumping jack , also you can filter the excersises just go to workout and click on filter and you can select back or butt excersises there itself ...
Julia's got the right idea, supermans works great. HIIT (high intensity interval training) is great for fats loss. Also, checkout ab-workouts-guide.com there's some good lower back exercises like the reverse snow angel, the wheel, and corkscrew. They're a little tough but well worth it. Look around Skimble you can to find there are some good lower back exercises like hip ups, the single leg floor lift, and side planks.
Good luck and God Bless!