Hello!
There is a super secret sandbox in the game, and it doesn't require you to edit the save file or anything strange like that. Simply hold the space bar while creating a new game until the transition to the map is complete and voila, items for days. As the game developer though, I will warn you that playing the sandbox takes the fun out of discovering new items in the game and figuring out what they do. But, chances are you have probably already found all the new items since it looks like you've been playing for a while and gotten to the final boss.
Hope this answered the question and enjoy!