I am particularly taken by two passages in the account which deal with specifically pre-Christian customs as recorded by Al-Ghazal. The first remarks that although the Vikings of the mainland are Christian, their fellows in the outlying islands are still Heathen (which is one reason the account could more easily be speaking of Ireland than Denmark; at the time the Danes were still firmly Heathen).
They were heathens, but they now follow the Christian faith, and have given up
fire-worship and their previous religion, except for the people of a few
scattered islands of theirs in the sea, where they keep to their old faith, with
fire-worship, the marriage of brothers and sisters and various other kinds of
abomination. (from p. 20 in the above-referenced pdf file)
We obviously can never know what the "various other kinds of abomination" were (more's the pity!), but the reference to fire worship is naturally of interest, as is the apparent custom of sibling marriage.
The first might make sense as a Muslim interpolation, since the term used (majus) is the same used to describe the Zoroastrians with whom they were already familiar, and whose propensity for fire-worship is well-attested. However, it is also the case that the custom of lighting bonfires at holidays is one that is widespread among Germanic cultures. The fires in celebrating the spring and midsummer are well-known, and are of particular interest given that the embassy would have been traveling during the summer months and could very easily have witnessed such things first hand.
The reference to sibling marriage is somewhat less clear, as the taboo against such thins within Germanic culture is well-known. That an outside chronicler would choose that particular custom to mention seems somewhat problematic, given that it is at odds with the other evidence. It could, however, be a reference not to a marriage practice among the Heathens themselves, but rather a somewhat muddled reference to the practice of marriage between divine twins in Heathen mythology. Frey and Freyja; Njord and Nerthus; these are more likely candidates for the reference to my mind.
The second interesting passage (from the standpoint of the religious reconstructionist) is this, where the Viking queen is speaking of their marriage customs and sexual mores:
'We do not have such things [the ambassador's companions had warned him not to spend so much time with the queen, believing it to be scandalous] in our religion, nor do we have jealousy. Our women are with our men only of their own choice. A woman stays with her husband as long as it pleases her to do so, and leaves him if it no longer pleases her.' It was the custom of the Vikings before the religion of Rome reached them that no woman refused any man, except that if a noblewoman accepted a man of humble status, she was blamed for this, and her family kept them apart. (ibid, p. 23)
Setting aside the queen's reported notion that Christianity somehow eradicates the emotion of jealousy, the notion that Heathen Viking women were so deliberately wanton (within their own class boundaries, of course) is one that I have not hitherto come across in my own reading. This could, of course, simply be a case of the Muslim author attempting to paint a picture of the libertine Heathens as a counterpoint to the chaste and modest women of his own land. However, it is not something that can lightly be set aside, and could probably be better understood in the context of a more general study on women's roles in Germanic society, legal codes regarding marriage and divorce, etc. Such a study is beyond the scope of this post, but perhaps it will spark some new connection in one of my readers.
On the whole, I recommend a read of the source text of Al-Ghazal's Embassy as an interesting and somewhat little-known source. A view from the outside looking in, refreshingly not from a Christian perspective.