I came across this question and tried many suggested answers. In the end, I found Lightadmin. It does not generate CRUD code, but provides a nice front end to Entity classes with very little code. To make the User entity available through the Lightadmin web interface, it only needs two changes in the XML files and then a UserAdministration class:
public class UserAdministration extends AdministrationConfiguration<User> {
/* empty */
}
You can configure the field names (and probably much more) in this Administration class, but I find it already very useful without any further configuration. The Getting Started guide really shows how easy it is. After these simple steps you can open the admin interface over http://localhost:8080/TestApp/admin.