Is there a new game plus mode in Enhanced Edition? Or any sort of post-game content, besides the expansions?

You could, of course, just start a new game and import your old characters.

You might also want to look into Heart of Fury mode, which greatly boosts the strength of enemies, to the point where the party you cleared the game with will still be challenged. (Also, with the higher difficulty comes higher XP, which will allow you to continue leveling up.)

Note that this is also included in the Classic Edition (assuming you have Heart of Winter installed), and in Icewind Dale 2.

Thbaks for the informative response. I was reading that the max level in icewind Dale is level 30? Does that ever go up?

No, it won't increase past that, but after a while there isn't anything new and interesting to gain.

Also, multiclass and dual class characters are limited per class, not total, so you could eventually get your fighter/mage/cleric up to level 30.

By the way, note that Heart of Fury mode affects the usefulness of certain spells. For instance, summons are really useful (they get boosted just like enemies do), and Power Word spells don't work well because the enemies have too many HP.