No one can resist food and drinks. It’s always a fail-proof way to draw folks from every department into the same room. I’ve decorated the office with balloons and streamers, ordered some branded swag with the new product name or swag that is somehow related to the product (i.e. for a cybersecurity product, we’ve done those little camera covers for your laptop), and made custom word searches or crosswords with key terms related to the product (raffle off gift cards for people who complete them).
While you have everyone in the same room, it’s a great opportunity to share some key information about the new product and the opportunity in the market, have leadership say a few words about what this means for the company, give some recognition to the teams that were involved in developing the product and getting it to market, and share any early customer quotes or anecdote to hype everyone up. I also like to have the sales lead announce any new sales incentives or friendly competitions we have in place for the launch (i.e. first sales person to collect 3 customer quotes about the new product gets a prize).
Good luck and congrats on the launch!