Maybe you could be honest with her and tell her you don’t want to lie but you don’t want to tell her cause it’s a surprise. A good surprise. And that you’re not going to say anything else, but you do have yarn and you’re working and it’s a good surprise. That would be clear enough that she’d get it and it wouldn’t dishonest at all, but you wouldn’t be disclosing any details of what you’re doing, so still some surprise remaining.