"Honest to goodness" is a common phrase used to express sincere and genuine assurance or affirmation. However, there are several other similar phrases that can be used instead. For instance, "honestly speaking" or "truth be told" can replace "honest to goodness" without losing its essence. Additionally, "in all sincerity" or "without a doubt" can also serve to convey the same notion. "I must confess" or "in all honesty" are other possible alternatives that can be used to express genuine commitment and transparency. Giving your word, promising, or guaranteeing certain outcomes could similarly capture the meaning of "honest to goodness." Regardless of the phrase used, sincerity and authenticity should be prioritized in communication.