You say it's an engineering firm? How about supplying a bunch of odd-ball halloween-themed items, allow employees who want to play divide themselves into teams, and challenge them to build a Rube Goldberg device for delivering halloween treats?
I work for a biotech company. Most of the employees have a minimum of a BA or BS in some area of biology, engineering, or chemistry, and a large majority have advanced degrees. At one of our office parties, we played "Minute to Win It" type games, and everyone had a blast. The games we played were balancing 5 dice on the end of a tongue depressor ( a LOT harder than it sounds), bouncing pencils into a cup, shaking ping pong balls out of a tissue box, and stacking and unstacking cups. You can look online for more games. People were given the option of signing up to play or just watching. Like I said, everyone had a great time -- watchers and players alike. At another party, we played hot potato, which was a hamster ball filled with candy (the winner got the candy). As juvenile as it sounds, everyone had a great time. It was amusing to see how competitive people got. We enjoyed it so much, we played twice!
Believe it or not, adults sometimes love to have the opportunity to act like kids! Have fun, and enjoy blowing off some well-deserved steam.