I like workout balls personally. Anything you can do that uses one makes the workout much more effective and works way more groups than it normally would.
Like those half ball things with the platform bottoms, those are really versatile.
For instance for the guys, if you want to make your push-ups really difficult get one of those half workout balls and flip it upside down then put your legs on some incline surface like the top of your bed or something, then hold on to the sides of the platform and do push-ups like that. If that's too hard you can get a full ball and put your feet on it and just use the ground with your hands. If you want it even harder though you can combine the two. You could flip it upside down and do squats and stuff too.
You can actually do side crunches with a workout ball too. Cardio stuff is a lot more effective with it as well.
I think one of the most effective things you can do for your abs is just barely sitting on the edge of something sturdy and having a weighted ball between your ankles then raising your legs up. You should get to where you don't need your hands on the chair or bench to support you from falling over, but you can do it to help yourself out at first.
Another similar one I really like is laying flat on your back then lifting your legs straight up into the air vertically and then your hips, then your stomach, and you can even go all the way straight vertical to your shoulders and then slowly back down. That one's fun.
A real easy one is if you could like lay over something like say over the edge of your counter with your legs on the counter but the rest of your body hanging off and then you have a weighted ball in your hands then you bring your body all the way up straight horizontal then back down. It's a lot easier if you have something designed to do that though with like handle bars beside where your upper body is otherwise getting out of that position may be less then graceful lol.... You could use some stools or something to support your arms to help lift yourself up onto your counter when you're done, barring that.
I dunno dude, like... I mostly do cardio and butt exercises so I don't know all that many exclusively for abs. Most of the ones for your butt will inadvertently work your abs too since it helps to tighten your abs/obliques/lower back and your legs/thighs, because it helps tighten everything up and push it out.
