What could be wrong with a grape? It is best to avoid eating a complete fruit or vegetable that is less than a kezayis. There is a dispute whether one may need to make a bracha acharona even if one ate less than a kezayis, since she ate a whole fruit. If one eats more than a kezayis she should certainly make a Bracha Achrona. If one does wish to eat just one grape, a solution would be to bite one half at a time.