What's the easiest way to create CRUD interface for custom entities?
The Entity Construction Kit (ECK) module allows you to create and manage entities through admin.
Here is a quote about it (from the module's project page):

...  builds upon the entity system to create a flexible and extensible data modeling system both with a UI for site builders, and with useful abstractions (classes, plugins, etc) to help developers use entities with ease.
ECK allows the creation and management of entity types with custom properties; adding bundles to entity types; and fields to bundles, with the help of the Field UI module.