We brought Aero chocolate bars from Canada on our last trip, and left them in a ziplock bag with the tip money, labeled, "For housekeeping." AFAIK, Aero chocolate bars are sort of unique to the USA, and housekeeping did accept our 'tips.' I found though, if we left money out with a note saying 'For Housekeeping' it wasn't the same as sealing it up in a bag, to make it known what exactly we were tipping.
Our fancy Mousekeeping envelopes, though looked cute, were never opened, either.