Standing Hamstring Stretch
This stretch can be done in a couple different ways. The most basic one is standing with your feet apart and bending over at the waist until your feel a stretch. Then just hold that position and relax as much as possible.
Avoid reaching or bouncing at the bottom, because that puts unwanted stress on your low back. This version can stretch your hamstrings and low back if they are tight. The straighter your legs, the better the stretch of the hamstrings, but avoid having your knees locked/hyper-extended.
The second variation is to stretch one leg at a time and have the leg you are stretching elevated on an object. You can use things like a chair, couch, step, table, or whatever you have available. This lets you focus more on each hamstring and provides a greater stretch
With either variation, you can add a calf stretch by pulling your toes towards your knee. When adding the calf stretch, only stretch one calf at a time.