Honoring In-laws

So I have an idea and I am not sure how to make it happen. My FH's parents have both passed :( but I want to make them part of our day. We have a few things planned for the ceremony but I really wanted to do something just from me.
I love the plaques, signs etc that say :Thank you for raising the man of my dreams' so what I want to do is to go to the cemetery the morning of the wedding and to leave something with this saying at the grave. My question is what can I leave. DO people leave signs and such at gravesites?
Thank you for any help.

I think that's a great idea!

As rcotter90 said, make sure that the signs are weatherproof. We've had my grandfathers headstone decorated 24/7/365 for the last 12 years.

Just keep in mind that many cemeteries will have a maintenance crew that comes around every so often and clears things off the graves. If you don't want to lose the signs, check with the office where they are buried to find out the date of the next clean up is - making sure to remove them before then!