You can do whatever you want, however you want. We had a relatively formal affair without a celebrant; a welcome/what is a naming ceremony speech from my in-laws, a poem and speech from me and DH introducing our mentors, then a finishing speech from my parents and a champagne toast.
With DS it was in winter, so there was hot drinks on arrival for 20 mins ish, then ceremony which lasted about 20 mins, then a hot buffet lunch (lunch bags for kids) and general mingling for about an hour, then people headed off as a lot of them were travelling a long way.
With DD it was summer, so we did it in the afternoon - people didn't mind driving back later as it wasnt dark) and after the ceremony we had afternoon tea (still party food bags for the kids).
We asked people to write advice in a book for DS, and something similar for DD. At others I've been to, I've seen people ask guests to bring a favourite book to start a library, to decorate a precut square which was then made into a quilt, to donate to a lifetime membership of a charity, to predict their favourite subjects/future career, and to pin notes on a map telling them places they should visit.
What I love about naming days is that they can be really personal and reflect what is important to you. Enjoy!