Diagonal Lunge
This is one of my personal favorite lunge variations, and it is so much better with the tree. The stretch in the side body and in the hip flexors can get incredibly deep. You can create so much space in the lateral chain, and you can really feel the stretch all the way from head to toe.
Begin on the side of the tree, with the hand of the inside arm on the trunk. Keep whichever leg is closest to the tree planted down. Take the outside leg back, and diagonally across towards the tree. Bend the lead knee closest to the tree, and slowly drop the hips down. Next, keeping the spine elongated, side bend towards the tree — taking the hand of the outside arm to the trunk if possible. Stay here for a couple of breaths. Then, drive down through the lead heel on an exhalation to return to standing. Practice on the other side as well.