When DH was living overseas while we were still dating every time I visited I'd hide notes around his apartment. I hid them everywhere! One time I hid about 100 little notes in all sorts of places... sock drawer, utensil drawer, under his computer keyboard. It would take him at least a month to find them all sometimes longer.
It was something he really enjoyed since he would find them and they would brighten his day.
One fun thing is to write a message if you have shower doors. Use a bar of soap and a light touch. When they fog up during a shower you can read the note!
Post what you decide to do I'm sure it will be fun and thoughtful!