All you need is a small bottle with a cork and a small piece of paper. Write a special message on the paper and stick in the bottle. Then place in a spot your loved one will see it.
You can also embellish the bottle or write your message in a neat font, but I like the simpler, handwritten version myself.
I found a bunch of these a few months back at Anthropologie. At $16 each, I thought that was a little crazy. You can certainly make this for literally less than $1 per message.
Save-On-Crafts has a bunch of options for the bottles. There are several lots of 12 where each bottle is less than $1. Also try your thrift stores and craft stores.
I snapped this picture on my phone of an example I saw while I was in the Anthropologie store. I love how they used some twine to wrap the message. It makes the paper more compact and easier to remove from the bottle.
Nothing like a daily love note washing up on your bedside table each morning!