Purchase a pack or two of disposable diapers. You will need one or two (more if you like) for each guest at your party, so buy enough packs to cover it. Infant/Newborn size is fine as mom will most likely go through many packs of that size before baby outgrows them.
Setup a table or area with the diapers and a bunch of markers and pens. As guests arrive tell them to decorate a diaper or two with a funny note, a drawing, a special thought (anything they want really), but something that will bring a smile to the new mom or dad’s face during those 3am diaper changes they will be doing.
At the end of the party put the diapers back in to the package and give them to the mom-to-be as a gift. Make sure to tell her to not read or look at them until she uses them, that way it is a nice surprise and something to look forward to with each changed diaper. If you want to really spice it up as a gift you can put the diapers in a new diaper bag with some baby powder and baby lotions. It is a unique and fun activity that will be appreciated.