Exercise is a weird thing, I personally don't need any motivation, I guess I am just so used to doing it by now.

As for core muscles, just planks and crunches.
Crunches are weird, for those that know, it's only the first few sets that hurt, after that the pain is gone and you can keep going, perhaps a doctor or fitness guy can explain.
I am now at 270 crunches for tonight and will go up to 400.