You seriously can switch it up however you like. for chewier bars, use 3 cups oats and no rice krispies. I usually throw in a little vanilla, peanut butter, sesame seeds, any other chopped nuts, a couple tablespoons of brown sugar, a sprinkle of cinnamon, maybe some coconut or raisins.
mix all together.
grease a small cookie sheet and flatten down (if the pan is too large, just use half of it, the bars won't spread).
Press down firmly.
Bake on 325 degrees for about 20 minutes (just browned around the edges). Bake longer if you prefer crispier bars.