If you haven't completed the High Hrothgar
quest yet, you can still travel to Solstheim by seeking out passage at the docks in Windhelm. Once there, you will have hours and hours
of quests to play through, even without
the main quests. There are NPCs who need help with some things, wild creatures and bandits roaming about, places to discover and, of course, dragons to kill.
You can go into business with an prospector to uncover the riches of a barrow, help (and gain the friendship and assistance of) a powerful wizard who lives in a magical tower made from a single mushroom. You can also get in good with the locals at Raven Rock by helping to solve their little problem with "Ash Spawn" - undead (and angry) souls who rise up from ash into vicious humanoid monsters bent on killing the living.
There's also a loan shark and general ruffian, Mogrul, to whom almost everyone in Raven Rock is in debt. He threatens everyone - when the guard isn't around - and will send henchmen to collect on his debts, but won't do anything "wrong" when the guard can see him, so you'll need to do your best to avoid him or use your choice of diplomacy, intimidation or stealth to deal with this character.
My favorite aspect of Dragonborn (and Skyrim, for that matter) has got to be simply the exploration... and Dragonborn provides a lot of it. One thing that seems inconsequential but that I spent three levels on (well, really quick levels), was simply a hollow tree stump in eastern Solstheim. I affectionately refer to it as the "Horker Dojo." It's perfect for practicing (and majorly leveling) Sneak, Conjuration, Heavy Armor, Light Armor and Restoration, but I'll give full details about that in an upcoming post. Check back for a link below.
If you're looking for more Skyrim, I highly, highly recommend Dragonborn. Hours and hours of additional content await you. Again...