Create a display table with pictures and things that represent the day the child was born. If it is an international adoption, do a theme from that country.
The guests invited should buy a gift of clothing, toys, etc. couple adopt a child it's an event to introduce that child to their adopted relatives and family friends.
In the adoption agency that I work at it is common to give the child a "rebirthday" party to celebrate their joining of the family, and do so annually thereafter.
