Unfortunately, while you say you have a small stomach, if you cannot see your abs, you are not lean. Leaning out is up to your diet, not exercise.
The simplest thing to do if you want to lose fat and lean out quickly (this is assuming your entirely healthy and have nothing like diabetes or anything else ) stop eating all the foods with sugar in them.
Check the labels, nearly everything you buy packaged has sugar in it. If you can manage to cutout sugar from your diet, you'll lean out in a couple of months naturally.
Reducing carb intake (you need to up protein and fat to compensate for loss of calories) will accelerate this process. Again, assuming your healthy, your body doens't actually need carbs or sugar. It's perfectly capable of making glucose from fat.
If you really want exercises that are good for ab strength, consider squats and deadlifts. Contrary to what our culture would believe, your abs are for keeping intra-abdominal pressure at a level that compensates for the stress placed on our spine when we lift and move. In other words, they force your guts against your spine so you don't snap when you lift things.
They do not make you bend over, though they do assist with that motion, and are involved in helping the pelvice move among other torso motions. However, their primary function is to keep your guts in place, and support your back.
Thus lifting exercises that force you to tighten your whole body to complete the lift will greatly strengthen your core.
- overhead pressing of any kind while standing
- olympic lifts
- and anything like these