Mount and Blade: Warband is a pretty amazingly engrossing sandbox game. I didn't realize it until the second time trying it though. First time, I played the tutorial for about two minutes before deciding it was the ugliest, jankiest RPG in recent memory. Glad I gave it another go several months after that.
Best way I can describe it is as a way more hand-ons approach to Total War (Medieval, specifically), where you take direct control over your commander, yet still have units at your command as well. Plus RPG elements, skills, gear, etc.
I hope Kenshi ends up being as good. And that the in-development M&B 2 refines some of the more janky aspects of the game.