Here's a super efficient body weight exercise that you can practice anywhere. No excuses!
Ideally you lift yourself from the ground and hold the position: straight legs in 90 degree angle to the torso. You can also lift your legs closer to V-shape, but that seems totally impossible, to me at least. It takes time to master L-Sit, it's so hard. On the top video I'm demonstrating the pose although I can't hold it very long with straight legs.
The way I've practiced this move is I lift my bum up as high as possible and try to bring it behind my arms keeping my feet on the ground at the same time holding the position as long as possible or doing sets of 5-10 repetitions. If you have a foam roller you can keep your feet on top of it and do the same thing. It takes some of the weight off. When you get a little stronger you can start lifting one leg at a time and then slowly both, first knees slightly bent and little by little straightening them.
L-Sit works on your entire body especially core and arms. Give it a try!