The hardest area to keep flat when you on holiday! You can target abdominal muscles best by performing exercises like lying on your back, wedging a yoga block (or even a shoe) in between your thighs, raising your ankles a foot off the floor, and holding that position for as long as you can. Raise your torso too and place your hands behind your head as for sit ups, and you’ll be pinching the object in between your thighs, while also directly tensing your abdominal wall for a minute at a time for ten minutes, twice a day.
Consistency is key! It may take weeks or even months to finally flatten and chisel that tummy, but consistent practice will pay dividends for life. Abs Workout is a great Android app with a host of info too if you really want to push things. Keep up the leg-raising, block-squeezing routine though, and add an exercise or two on top every week from the app.