Push PoD to 878 and crushing Alfred the Great in Wessex, then the rest of England falls for good. You know, more or less.
Dublin was founded in 841 by the Vikings themselves, and was a huge slave market due to Ireland being their bitch. Just give them that extra push to break the tuatha into easy to swallow bits. If Alfred the Great falls, England remains a focus, and Ireland gets more attention.
Dal Riada destroyed in 839, the Picts conquered by the Scots a couple of years later. Perhaps formalize an alliance with the devastated Picts prior to this so that the Scots are caught between hammer and anvil. Not destroyed, but kept disunited and a Pictish element as a Danish puppet/ally. Over time, pull 'em down in full.
From 850-875, Iceland begins to be settled.
Rurikid Dynasty established in Novgorod in 862, a Varangian dynasty, the rulers of Kievan Rus to Grand Duchy of Moscow to Tsardom. Form a strong and lasting Dynastic Union. Vladimir I baptized to Christianity in 988.
863-879 East-West Christian Schism; do some fancy footwork and have larger swathes of Northeastern Europe go Catholic; perhaps with that Rurik tie, predate the Northern Crusades of the Baltic states. (Final Schism not until 11th century, and that's assuming Catholicism follows the same basic path.)
900s - Normans, though if Alfred the Great is defeated and Britain stays Danish, the Normans might not 'go native' and Denmark retains a strong hold. Maybe keeps them likelier to stay a vassal... maybe. Not sure what that means for relations with France later on. Also would preclude Norman conquest of England. At least in the "Hastings" sense...
**(Possibly, but harder, to retain relations with Italian Normans. Truthfully, not really sure how the Normans were related to Danes/Vikings. If relations remain, opens up much Mediterranean opportunity for Danish shenanigans.)
1001 - Vinland, Newfoundland. Leif Eriksson. All that jazz.
1035/1066 - Avoid ALL breaking of the empire between claimants, heirs, rivals, etc, throughout the history, such as witnessed with Canute's death and that of Edward the Confessor. This is also assuming some guy doesn't go "I'm King of Scotland now" and seek to break away. Or at least not very well, maybe retains puppet/vassal status.
1095-1099 - Danes join Crusades, pick up some... something. I dunno. Gives them a foothold down there for trade, tacked on to the via-Russia-route. Including Constantinople, which by this time is freaking out due the Seljuks. Always in need of good Varangian mercenaries!
1107-1111 - Another round of Crusading fun. And in 1145. And... you get the idea.
1147 - Wendish Crusade in N.Germany. Of course, this might have been done earlier if the Danes were more religiously motivated or gave a care for more continental dealings. (Same as Livonia, etc)
1204 - OTL Latin Empire formation; butterflies could change that to a Danish-related one earlier. I mean, not Copenhagen-ruled, but related somehow, as I mentioned Varangians ealier.
.......
Meh, whatever, I'm sleepy.