The Shulhan Aruch (O.H. 291:5) rules that one should have bread with Seuda Shelishit but if not possible, one may use food that was baked with one of the five grains. Furthermore, one could use meat or fish in place of bread and there is an opinion that one may even fulfil Seuda Shelishit with fruit. Nevertheless, the Shulhan Aruch repeats that the foremost opinion is the first one and that bread is ideal, unless one is already satiated. In Morocco, the common custom was to not necessarily have bread during Seuda Shelishit, but rather cake, fruit and the like.
When asked personally, Rabbi Shalom Messas answered that he tended to still feel full even at Seuda Shelishit and therefore avoided bread. Thus, one should plan ahead when eating on Shabbat such that one will have an appetite for bread during Seuda Shelishit.
Regarding other foods which should be consumed during Seuda Shelishit, the Kaf HaHaim (K.H., O.H. ibid:29), quoting the Pri Etz Haim, writes that one should make a point of having fish.
Rabbi Haim Benveniste (Knesset Hagedola) writes that since Moshe Rabbenu died on Shabbat, as a sign of mourning, one should eat eggs. The Magid Mesharim (Parashat Tzav) writes that one should always have at least a Kezayit of a cooked dish for Seuda Shelishit.
Summary: One should wash for HaMotzi for Seuda Shelishit and should eat a cooked dish, such as fish, with this meal.