Yep, just plain old push-ups.
Push-ups are done by lying on your stomach and pushing your core up and down (slowly ).
Sit-ups or crunches whichever you prefer, personally i use sit-ups.
Sit-ups are done by lying flat on your back, bend your knees 45 degrees (there should be a foot long space between your heels and butt) and lifting your shoulders up to your knees.
Muscles being worked;
Abdomen (Stomach and Core)
Its harder than you think.
Front Plank is done by lying down on your stomach and holding yourself up by your elbows (like the picture).
Muscles being worked:
Abdomen (stomach and core)
Erector Spinae (lower back)