You could bring a "putting green" to the office and have a putting playoff for your husband and his co-workers. Make it a lunch thing with sandwiches for everyone and a cake. If it's a fun office environment, that could go over well.
Otherwise, maybe you could find a character that could come to the office and do some sort of singing telegram. It's so corny, but it could be cute if you find the right character. Or a clown or magician to make co-workers and him balloon characters - including a balloon hat for him...
Aside from work gatherings, something simple like a bowling night could also be fun, low key, but active and a good time for everyone. Depending on weather and your location, you could do frisbee golf.
Hope you have a great time! Happy Birthday!