Constant instance variables (or 'instance constants'? :p) would be terrific in their own right. Even so, I think traditional 'Enums', for me, excel at 2 important things that make them especially worthwhile compared to other approaches:
1. Forcing a single choice. An enemy can't hide and attack at the same time. In code, instead of having to specify a priority order, one could simply select 'State.HIDE' from the list and call it good - super clear and less prone to buggy logic - like using a Boolean instead of isTrue and isFalse vars (hopefully no one has ever done that). Plus, the idea of a neat little drop-down in the C2 user interface when testing against an Enum really excites me.
All in all, I think Enums would be an awesome addition to have down the line, but I see why this wouldn't be an immediate priority because there are a handful of alternate approaches.