Bulgarian split squats:https://www.youtube.com/watch?v=lfIh-Uilfq0
Lower body stretch:https://www.youtube.com/watch?v=dOw8F0YITUw&t=66s
There are studies that show the number one predictor of low back pain is actually insufficient core strength. Because of this, and because we tend to extremely under-utilize our core, I tend to choose things like Bird dog and Dead bug where you can start small and work your way up.
Bulgarian split squats and glute bridges are a popular choice because, since most people sit all day for work, they end up getting weak glute muscles over time. Bulgarian split squats help strengthen each glute muscle individually. Do with or without weight. Glute bridges strengthen both glute muscles at the same time, and some progressions allow you to strengthen each glute singularly.
The lower body stretch chosen above is also a great choice since most people sit all day. Often, when we sit for extended periods of time, our hip flexors are short and tight. Holding this stretch should help relax those tight muscles and improve posture, lower back pain, and hip pain.