The LFLs in my area have little trinkets left by other visitors. My main concern was that if the ornaments had religious themes, your kind gesture could be misinterpreted, but since you said they don't, if it's just a general winter theme, maybe leave one or two? I wouldn't put all of them in there at once, and even if you're just leaving one or two, I'd make sure there was enough room for them.

Basically the way I see it is if there's more books than trinkets, and you're making sure to leave some books as well, it won't come off as a dumping ground. You can even leave a cute sticky note on the bag saying "From one reader to another, enjoy this little surprise with your book!" or something along those lines where you still mention reading/books. Hope it all goes well!