You can *absolutely* keep playing after the end of the main scenario! In fact, the retire feature lets you end the game at any point, even before concluding the main questline (don't expect a very good ending if you do that, though).
There's some content that we put in that is definitely intended to be a challenge for the hardest of the hardcore, including something somewhat spoiled by a certain preview of the game.
I reached level 16 in my playthrough and I could have easily hit level 17 if I had wanted. I did not complete all of the content, either. I don't think it's unreasonable to hit level 20 if you do absolutely everything and do a lot of random encounters, but I think that the high teens are more likely where you'll be at if you complete all the content and don't bother grinding.
One thing I will say is that a LOT more of the content is optional in SoZ than in any other NWN game to memory. I would say probably on the order of 20% of the content is on the crit path and 80% is side content - we're more in line with Fallout in that regard than NWN2.
Of course, if you want to actually be able to DO the crit path stuff, you better do at least some of the side quests... Much like fallout, you *can* skip a lot of stuff but you'll find the XP/loot will come in handy. Not to mention it's fun to explore.