After about 250 hours of game play, I've completed all of the quests I can find on the maps for Skyrim and Solstheim.

Is that it? Are there any more lands to explore? If so, how do I access them?

Not without mods. In the vanilla game Skyrim is surrounded by invisible borders that cannot be crossed.

Dawnguard takes you beyond the borders of the map, although not by much.
It also has a realm of Oblivion to explore.

it is Skyrim and Solsthiem only.

250 hours in and you have completed all quests Impressive...I have over 4000 hours in and I've yet to do " all quests ", you sure that you have done everything ?

But to answer your question...... No, Skyrim and Solstheim is all there is excluding the odd quest that sends you to another location like Skuldafn, Soul Carin, Sovngarde.

Soul Cairn from dawnguard, but that's about it.

I was a little disappointed with the Soul Carin, was hoping to face the undead army or something and find some pretty good rare items. Boy was I shocked once I actually went there

The Soul Cairn was just annoying and a pain in the ass to get through. I was so happy when I finished that part.

Me too It was a wasted opportunity, IMO. There could have been a hell of a lot more undead things trying to kill you at the very least, I think I counted around 30ish hostile NPC's killed by my sword

The Forgotten Vale.

No thanks. More things to kill wouldn't have made the Soul Cairn any better.

Good idea for a plane of Oblivion in theory, but poor execution. I don't even bother with the side quests there.

No,no I don't mean just more killable NPC's. Just at the very least they could have added more hostiles to the Soul Carin, you don't want to get me started on my ideas as to what could have been done..... It's the plain of souls and undead for christ sake

But alas it was a waste of space, a missed opportunity. Which is why I don't even bother anymore to search out all the unique stuff there, I push through that part as fast as I can just to get it over and done with