Is it permissible to return the monetary value of a condom taken from a father without his knowledge, instead of returning the item itself, especially since informing him would be difficult and embarrassing, and knowing that the father will not demand it back? And is it obligatory to inquire about the owners of forgotten items or borrowed toys from over ten years ago in order to return them?

The questioner is suffering from obsessive thoughts and should disregard them. What the questioner mentioned is not considered mithli (fungible/standardized) by many scholars, for mithli is that which can be measured by volume or weight and in which salam (forward contract) is permissible. Therefore, if anything is due from the questioner, it is to return the value of what he took, not its equivalent. He is not obligated to explain to his father the reality of what he did. As for the toys, if they were a gift or it was customary to overlook such matters, then the questioner owes nothing to their owners.