I absolutely think you can give them something no one else can - your help and expertise as a designer, and labor.

Get a nice piece of heavy-weight card-stock, and create a "room redo-help" certificate, including help with design choices, paint and physical labor - as in painting. Now, to make it look like a traditional wedding present, get a gift box, fill it with pretty tissue paper, and place the certificate inside. For weight, add a pretty, wedding - or car-themed paperweight. Or "his and her" Matchbox cars. Then you have a nice box to wrap, a gift that is fine for them to open, and even if you and they know the "room" in question is the baby's nursery, it isn't publicly mentioned. And the paperweight is something small but useful, and personal, or the matchbox cars are just fun desk toys until a couple years from now, when baby can play with them. But for now they are grown-up toys.