exercises that work abs??? especially lower abs plz?

I really want to work my abdominal muscles. I know that different kinds of crunches and sit ups help, but I only know about the traditional crunch and would like to learn how to do the different kinds like reserved crunch etc. However, any alternative for crunches and sit ups is perfectly acceptable, too. And also I would like to learn some exercises that work side abs and lower abs because crunches and sit ups just don’t seem to touch some parts and my upper abs are more developed than my lower abs, so I especially want to work them. And, last of all, do you know any cardio exercises that I can do indoors? I just can’t do outdoor cardio like running, walking etc., don’t ask why, and I can’t take classes because I don’t have enough time and cash. Thanks.