Ask For Hand-Me-Downs and Heirlooms
Forget pink and blue: go green! Ask guests to bring hand-me-down clothes and baby gear their children or friends' children have outgrown, or an heirloom book, spoon, or special item they love, but are willing to part with for a while. Use the heirloom items as decoration in the nursery, but give it back when baby is old enough for a kid room.
Source: Flickr User mikewade