Also, 1 arm farmer's and yoke walks are good 'core' exercises.
Really just walking with anything heavy will do it, like the husafell stone or just a heavy sandbag in the bear hug position. It'll get some bicep work in, too.
Also just the stones, I remember the first time I lifted one and it felt like my abs were going to rip in half. I can't do it without tacky though and its a pain to clean up.