If I took a survey of 100 people in regards to what their favorite stretch was, I'd be willing to bet 75 of those individuals chose some type of hip flexor/Couch Stretch variation. While stretching the hip flexors might feel great, there needs to be some type of isometric component involved to ensure we're improving our mobility alongside flexibility.
Couch Stretch Progressive and Regressive Angular Isometric Loading (PAILs/RAILs) are a great way to improve both hip extension and knee flexion at the same time. Keep in mind that multi-joint PAILs/RAILs drills could be a bit extreme for beginners. However, when prescribed correctly, improving overall hip and knee mobility will lead to both feeling and moving better.