The easiest (and possibly tastiest) answer is nuts. All varieties of nuts: almonds, peanuts, cashews, pecans, hazelnuts – you name it. And for those of you who like it smooth, peanut butter is an excellent source of protein with 4 grams per tablespoon. Watch the salt though: the USDA recommends choosing unsalted nuts to keep the sodium intake low.
I believe the last answer is correct because this will enable you to measure in percentages the accuracy in order to decide when the accuracy increased.