yeah ... Denmark is probably one of the easiest choices in term of learning abilites ... they can do nearly everything good
The countries in Denmark is fairly strong, which means that you're able to strike above your weightclass, purely looking at country-count
You got a ton of Dynasty-members (Children) that can be merried left right and center (probably grabbing a few throne claiments as well while at it) for alliences
You are in a elective monarchy where at start there is only 2 voters, you and your eldest son ... and if voting is tied, the monarchs vote is deciding
You have a ton of arguebly easy targets to your east in the baltics and finland. Sweden is likely exploding in a civil war between Christianity and Norse, while Norway is to busy mucking about in England, and the Russian duchies are prone to infights or all ganging up on someone attacking them (most often from the steppes to the east or south)
You have a inheritable claim on England which you can build up to push (either by improving your lands, helpful allies or both)
You don't really have any obvious dangers making a play for your borderlands (Duchy of Saxony just south of the borders might make a play for Holstein but thats it, and it'll take a while for them)