The reservation app was super simple so no real guidance there
I'd recommend really getting familiar with the Django ORM so that you understand how it generates the schema and how to tweak it to get what you want.
As far as inventory management - I looked at several opensource projects and tried to understand their schema and use cases. Not just Django ones but other languages like PHP - PartKeepr for example.
Inventory management is surprisingly complicated to do in a generic way.