Skeleton Keys (jailor keys stack, so…)
All hearts (Grey One hearts stack, so…)
Sigils
Pet Eggs and babies (just average out the remaining decay times)
Recipes and Scout Reports (basically anything on a piece of paper)
All unique trophies and and harvested parts (lots of siege beast parts won’t stack)
Feel free to add to the list. There are just so many inventory items that take 1 items per slot.