Hi Mary Leigh! Sounds like a busy and exciting time! As far as a theme goes, I think it all depends on what your son likes. If he likes helicopters AND your husband is learning to fly them soon, it's a great crossover theme, but it sounds like a hard theme to find applicable use for.
What does your son like to play with? What are his interests?
It sounds like a good idea to have someone else makes the cake, with your mother-in-law being such a heel. I would go with 2 different cakes, they don't have to be large, but it would be nice to have one personalized birthday cake for your son and a grad cake for your husband.
I have a couple of ideas for activities that can involve both the kids and adults:
1. Scavenger Hunt - Have a different list of items for kids and adults and make the kids' items easier to find. Have different prizes for the kids and adults. You can do this inside if you have enough room!
2. Relay Race - Setup a obstacle/relay course with harder challenges for the adults. You can situate the course for kids to switch off to adults and back. This can end up with hilarious results if you're creative. Watch A Minute to Win It for inspiration!
Good luck and let us know how it goes!