I do the roman chair hyper extensions on Back day, and I will hold a weight to my chest. Like this
On the 45 degree roman chair thing, I warm up without a weight, and then grab a 25 lbs one for the next three sets, at 15 reps-ish. On the one like in the picture above, I usually go really light weight or no weight at all, and I still feel like my back is caving in the next day. Which is what you're going for, I assume!
I just started adding those into back day a couple of weeks ago and it has already helped my core, particularly in squats.
Here are more possible exercises, none of which I have tried other than the one I sais, under "Erector spinae" at the bottom this page