Waverunner, Some of my favorite alternate exercises are the following:
SUPINE RUNNER: lie on the ground with your head closest to the anchor point and TRX in single handle configuration. Place one heel in the foot cradle, extend both legs straight out and lift hips off ground to form a straight plane (similar starting position as the HAMSTRING CURL, but with body in opposite orientation). Drive working heel towards ground while keeping the non working leg straight out in front of you and in line with the rest of your body. As you push your heel towards the ground, you will drive your hips up. Continue to push your heel down, bending your working knee, until your weight is supported on your shoulder blades and your legs are oriented upwards with one leg bent and the other straight. (Your legs will be on fire after only a few of these!)
TWISTING HIP DROP: with TRX in single handle configuration, stand facing sideways to anchor point. Grasp handle in both hands, with arms high and elbows slightly bent. Drop your hips away from the anchor point while simultaneously twisting torso to face anchor point. Once at lowest point, immediately rotate up to starting position. (I like this exercise a lot better than the STANDING BACK EXTENSION, it seems to work my core a little more)
SUPER FLY: stand facing the anchor point, holding both handles in front of you arms out to the side with your elbows slightly bent. Lower yourself to a challenging angle. Keeping your body aligned, bring your arms together in a hugging motion. (a GREAT chest exercise)
There are a ton more exercises out there, I'm sure everyone has their favorites. Is there something specific you are looking to replace in the current workout?
"Teaching is only demonstrating that it is possible. Learning is making it possible for yourself." - Paulo Coelho