All meals that involve bread or mezonos must be eaten in the sukkah. However, if one is only eating fruits and vegetables, or less than a k’beitza (volume of an egg) of mezonos then a sukkah is not required11. It is meritorious to eat and drink exclusively in the sukkah12. One only recites the blessing “leisheiv ba’sukkah” if there is an obligation (i.e. a meal of bread or mezonos). During a meal, one may not eat anything outside of the sukkah, even fruits and vegetables13.
11. Shulchan Aruch (O.C. 639:2)
12. Shulchan Aruch (O.C. 639:2)
13. See Mishna Berurah in Sha’ar Ha’tziyun 639:29