With absolutely nothing to work with but gravity, you can do "flutters," lying face-down, spreading your arms out straight, and pulsing your extended hands up and down without letting them touch the ground. Doing this with your hands right by your side makes it easier to focus on lower traps, with your shoulders at 90degree abduction makes it easier to focus on middle traps, and with your shoulders in full flexion/abduction makes it easier to focus on upper erector spinae muscles.
In this last position, you can also flutter the hands in an alternating paattern (left goes up while right goes down and vice versa) while doing the same thing with the feet. This is commonly referred to as a Superman. A slightly more advanced version of this exercise, sometimes referred to in Pilates as "swimming," has the hands and knees on the ground, hands directly under shoulders and knees directly under hips. Keeping the transverse abdominis activated and trunk stable, you would then lift one hand and the alternate foot off the ground and extend them both out, aiming to reach a point where hand, shoulder, hip, knee and ankle are all in alignment.
You can also lie face down and do back extensions, using your upper back to lift your chest slightly from the floor.
Exercises such as the bridge work hamstrings and glutes.
With limited options, such as a sturdy railing or a strong tree branch, you can do various forms of pull ups and inverted rows, which would be ideal if aiming for increased strength.
Alas, because we don't know exactly how much equipment you don't have, or what your intensions are, it's hard to put a program/workout together for you.
