Some of the Scariest Goosebumps tales are the ones where the monster is something inmate, but has an eeriness about. A Evil Camera that causes accidents or Bad luck.
A mirror hidden away in a attic that turns you Invisible with dire Consequences.
Let's Get invisible! has a slow buildup to its reveal but ironically was the longest book in the original ten series, I think Stine really played around with some of the mythology of what this mirror was and funnily enough, He confirmed he Would Never murder Children in Goosebumps.
He sure did technically murder Lefty in this Book.