If you're looking for flatter abs The Plank is absolutely the best exercise for you. To perform a plank assume push-up position with your elbows bent, forearms extended on the floor. Your body should form a straight line and you should be contracting your ab muscles to stabilize your core. Hold for as long as you can, try for 30 seconds and work your way up to 2 minutes or more.
(Taking pictures is a good way to fix your form too, in this pic I realized my elbows should be slightly more aligned with my shoulders.)
Demonstration video below:
I am confident that if you do 2-4 planks everyday (or at least most days) you will see a difference in your stomach (they only take a few mins. a day). I can honestly say that even pre-pregnancy I never used to wear bikinis. I hated my stomach. Now since I started doing planks 2 years ago I wear them all the time, they gave my stomach such a toned, flat look and some nice definition :D. Do realize though that planks won't get rid if excess fat, if you're looking to lose fat off your stomach your best bet is reducing your calorie intake, along with a daily cardio routine.
I've been doing my planks on and off all summer, but I finally got serious this week and have started doing them every night again. I am very excited to see the results in a few weeks! As per my usual advice some great music will help you hold your planks longer, I know it helps me big time :)
Here's a video as well, preferably don't toss your hair around idk what I was doing: