Not sure what they’re called but on your back, knees up feet flat on the floor. Lift your pelvis up into the air. Exercises the back and puts load on your hamstrings. If you want to make it tougher do it one-legged.
Also spinning – when up out of the saddle, lean forward and you can really concentrate on pulling up on the pedals behind you focussing pressure on the hamstrings. Also good for concentrating your pedalling technique and getting a much smoother pedal technique which will benefit your cycling.
Ankle weights, gentle jogging on the spot, lifting your feet to your buttocks.
Lie on your front on the floor, get one of those big inflatable exercise/crunch balls and someone to hold it on your back by leaning in on it, then kick up your legs back so you hit the ball with your heels. Position the ball far enough up your back to the best position to make sure you get full range of hamstring movement. One minute of vigorous and fast kicking of the ball and you’ll definitely feel it in your hamstrings.