A special page should be created which acts as a sandbox for item pages: Special:ItemSandbox

This page can be used for test edits, the Wikidata tours etc. It would not actually save things to an item page but an api should be created instead which imitates the api instead of posting actual data to it.

This requires some javascript refactoring, but basically the html for an empty item can be added to the special page (maybe with the option to load the data from an existing item) and only the wikibase.api.RepoApi javascript module has to be replaced with a mock class.