The below 6.5 Minute video shares planks and plank-like exercises.  I know planks added to a regular Exercise or good diet plan can flatten your belly.  The video promises if you do the 5 exercises for 4 weeks you will get a flat belly.  That seems like a good deal for less than 5 minutes a day, if it works.  The idea is to start week 1 with about 2 minutes each day and then move up to 5 minutes per day in week 4.  Exercise included are: standard plank, left-side plank, right-side plank, modified sit-ups, and something they call backwards push-up (which looks similar to the old crab crawl from high-school gym class).  Most of the first minute of the video is just promo to get you to watch the rest, so be patient.

Always consult a doctor before starting a diet or exercise program!