My husband and I had another meal of beef and barley stew, with more of the Viking flatbread. (The number of dough balls in the bag doesn't seem to have gone down any, and the quality remains undiminished since the last time I fried some.) Tonight, he had his with maple syrup (very unperiod) and I had mine with apple butter.
Apple butter, though consisting entirely of materials that were available in period, is itself also non-period. The Food Timeline claims that similar preparations go back only to the Middle Ages, and in its present form apple butters are traceable to the Pennsylvania Dutch in the 18th century or so.
That certainly explains how I know about them, since we live close enough to Pennsylvania Dutch country that Amish folk come out to sell their farm goods in our local markets. But it doesn't really answer the question of what the Vikings might have eaten on *their* flatbread.
My husband had one plausible idea; honey. That seems likely to me. The other possibility is skyr, a very soft cheese that was closer to yogurt in properties and flavor; it's mentioned in the sagas. Probably they sometimes combined the two, perhaps adding dried fruit.
EDIT: I take back my comment about the period ingredients in apple butter. Apple butter uses sugar, which likely would not have been available in Scandinavia in period. It sometimes also has spices, including cinnamon, which I don't believe the Viking age Scandinavians had either. Nor do I know whether the basic means of making apple butter--i.e., simmering down seasoned, sweetened apples until they form a paste--works if you use honey instead of sugar. (Though it might be interesting to find out.)