Arthur's court and contemporaries had their own adventures to talk about, but only as these took place. What other material did their minstrels and storytellers use for an evening's entertainment; what other knights and heroes of history, legend, and literature did Arthur's people hav to feed their imagination?
Perhaps foremost, there were the heroes of the Old and New Testaments and of the Grail history: John the Hircanian, Judas and Symon Maccabeus, David and Solomon, Joseph of Arimathea, and so on. The basic tales of the Biblical characters seem to have been recognizable, (use a Bible with the dozen or so books which the Protestant tradition considers apocryphal and the Catholic tradition inspired), but other material had gotten mixed into the legends as well; besides such non-Biblical information as that found in Malory and the Vulgate (as, for instance, in Amide's tale of King Solomon's Ship), Arthurian folk might have retold such tales as can be found in "The Lost Books of the Bible" and "The Forgotten Books of Eden".
The Grail tradition as Arthurian characters were most likely to have known it can probably best be examined in Volume I of the Vulgate, Lestoire del Saint Graal. In addition, Arthurian folk surely would have had tales of early Christian saints and martyrs of the Roman and British martyrologies and the beginnings of "The Golden Legend" (a collection of saints' lives set down by Jacobus de Voragine in the thirteenth century), as well as others rememberd chiefly in local oral tradition.
For secular figures, they had the history and legends of classical antiquitity. "Sir Gawaine and the Green Knight" (Pertolepe) begins by referring to the Trojan War, Aeneas, Romulus, and others (in the Middle Ages, folks tended to symphatize with the Trojans rather than the Greeks). Seeing an artist paint the history of Aeneas gave Lancelot, while a prisoner in Morgan's castle, the inspiration for muralizing his own exploits. Very occasionally, as in the tale of Viviane's birth, we see a hint that the deities of classical and other non-Christian mythology may have been considered real, whether or not diabolic. Closer to Arthur's time were the heroes of early British history.
I [Arthur] have understood that Belinus and Brenius, kings of Britain, have had the empire in their hands many days, and also Constantine the son of Heleine, which is an open evidence that we owe no tribute to Rome.
The Heleine of this passage would be the same Saint Helena credited with finding the True Cross. A chronicle of early British history may be found in Spenser's "Faerie Queen", Book II, Canto X - Spenser's version of Arthurian adventure is probably irreconcilable with Malory's but, in a well annotated edition (like that edited by Thomas P. Roche, Jr., Penguin Books, 1978) his list of British rulers from Brute to Uther may be helpful to those who wish to go more deeply into the subject. There are other accounts.
Palomides, his brothers, and Priamus might have brought Eastern lore and legends to Arthur's England, if they still wanted to talk about such "pagan" things when they had determined to be Christened. Similarity, Urre of Hungary could have brought bits of Eastern European lore, and so on. As Arthur's Round Table drew knights from distant lands and became a symbol of the world, so Arthur's court may well have enjoyed a much wider range of culture and literature than we usually associate with the period.