Write out the letters A-Z on a piece of paper and then assign each letter a number, symbol or different letter.  Use this “secret code” to write your husband a sexy note.  Hand him the mysteriously coded note (with a mysterious smile, of course) and give him a clue as to where the decoding sheet can be found (brief case, under his pillow, to be delivered in the mail the next day, etc.).

Cf  hfofspvt!  Mpsj <><
(hint: my code is just to use the next letter of the alphabet ~ B becomes C, e becomes f, etc.  To break the code, just move each letter back one.)