Here are a few:
Double Leg Lifts: Lie down on your back, with your hands placed under your butt, with the palms facing the floor. Then, contracting your lower abdominal muscles, and tightening the muscles of your legs, lift them straight up vertically, and hold them there for a few seconds. Then lower them down until they are a few inches off the floor, and hold it there again for a few seconds. Repeat 5-8 times to begin with. Later, as you feel your abs getting stronger, increase the reps to 15-20 times. Then, as you get even stronger, you can lift your head and shoulders off the floor while performing the leg lifts.
Alternating Leg Walks: Begin by lying on your back, placing your hands under your butt, and take a few deep breaths. Then, contract the lower ab muscles and lift your right leg up slowly, until it is 90 degrees to your body. The, equally slowly, lower it back down, until it is a few inches off the floor, while simultaneously raising your left leg. Keep lifting your legs alternatively about 8-10 times to begin with. Later, as your abs become stronger, you can increase the count to 15-20 times. And, as in the previous one, lift your head and shoulders off the floor while performing the exercise.
Double Leg Reverse Crunches: Lie down on your back, with your hands under your butt. Lift both your legs up until they are in a vertical position. Now bend your legs at the knees, so that the lower part is parallel to the floor. Then, contracting your lower abs, and maintaining your bent knees, gradually lower your legs until the soles of your feet are just a couple of inches off the floor. Keep your abs contracted and hold this position for a few seconds. Then increasing the contraction in your abs, lift your knees up again, bringing them towards your chest. This is a somewhat challenging exercise, so begin by doing just 3-5 reps. Later, as you feel yourself getting stronger, increase the reps gradually, until you can do 15-20 reps.
Scissors: Lying on your back, with your hands under your butt, contract your lower abs and tighten your leg muscles and lift both your legs up, until they are about two feet off the floor. Now, without bending the knees, get your right leg across your left leg, so that both legs are crossed. Then open your legs out, and close them again, reversing the positions of the legs. Keep repeating this scissoring action, keeping your lower abs contracted, about 8-10 times, to start with. Then, increase that to 15-20 reps as you get stronger.
Double Leg Circles: Lie down on your back, with your hands under your butt. Then, contracting your lower abs, lift both legs a few inches off the floor. Now, rotate both legs together in circles. Make 5 circles in a clockwise direction. Then lower the legs to the floor, and relax for a few seconds. Then repeat the exercise with 5 anticlockwise circles. You can increase the number of circles to 10-15 as you get stronger.
See the video for a different version of leg lift!