[1] See Admur 188/10; M”A 188/7; M”B 530/1 that even eating fruits suffices, so long as one does not fast

Other opinions: Some Poskim rule that one is obligated to eat two meals on Chol Hamoed which consist of bread, and for this reason if one forget Yaaleh Veyavo he must therefore repeat Birchas Hamazon. [Tashbatz 214 brought in Beis Yosef 188; Nimukeiy Orach Chaim 530/1]

[2] M”B 530/1; Kaf Hachaim 530/4; Implication of Tashbatz ibid that one is required to have to have two meals a day on Chol Hamoed; See Nimukeiy Orach Chaim 530/1 that states that perhaps one meal suffices, although he concludes that according to the Tashbatz two meals are required.

The reason: As it is a Mitzvah to honor Chol Hamoed with food and drink, and the main part of a meal is bread. [M”B ibid] The reason that one is to have a meal with bread both by day and by night is because it is similar to Yom Tov on which we also have two meals with bread, one by day and one by night. [Shaar Hatziyon 530/5] However see Nimukeiy Orach Chaim ibid that explains why by Chol Hamoed it would suffice to eat only one meal as the only reason two meals is required on Yom Tov is because of the Kiddush obligation, which is not relevant to Chol Hamoed.

[3] Admur 188/10; M”A 188/7; M”B 530/1

It is for this reason that if one forgot Yaleh Veyavo in Birchas Hamazon of Chol Hamoed that he is not required to repeat Birchas Hamazon. [Admur ibid]

Other opinions: Some Poskim rule that one is obligated to eat two meals on Chol Hamoed which consist of bread, and for this reason if one forget Yaaleh Veyavo he must therefore repeat Birchas Hamazon. [Tashbatz 214 brought in Beis Yosef 188; Nimukeiy Orach Chaim 530/1]

[4] Elya Raba; P”M 530 A”A 1; M”B 530/1; Kaf Hachaim 530/4; Nimukeiy Orach Chaim 530/1; See other opinions above that hold eating bread on Chol Hamoed is an obligation

The reason: As it is a Mitzvah to honor Chol Hamoed with food and drink, and the main part of a meal is bread. [M”B ibid] And according to some Poskim [stated above] eating bread on Chol Hamoed is an obligation, and hence although regarding Yaaleh Veyavo we rule Safek Brachos Lihakel, nevertheless regarding the Biblical command of “Mikra Kodesh” one should be stringent and eat bread. [Nimukeiy Orach Chaim ibid]