Some adult activities would be nice too. Perhaps you can send out a questionnaire to find out what hobbies or interests they all have in common.
For instance, if they all do wine tasting you could plan a wine tasting get together on one of the boats on the water.
If they all like to go dancing you could host an adult date night with a light dinner and some dancing afterwards.
For family activities I would do a monthly picnic with you providing the meat, main dishes, etc...and "IF" they are receptive, you can let them bring some additional foods like potato salad, desserts, pies, etc... I would not want to make them feel "common" but special for being part of this. That is why I would consider providing everything. If they are a more down to earth group they will feel right at home doing pot luck. If they are more elitist they are not going to like that idea.
If they are more elitist then perhaps a cookoff: chili, spaghetti, BBQ, pie, something they can show off their skills and get bragging rights to.
You can do a monthly culture night, they can bring their favorite food from that culture and then you can plan a speaker, entertainer, movie, etc...that is germane to the idea. If an individual hates food from a certain culture they can do something else such as desserts, side dishes, and something for themselves to eat.
For example:
Italian night:
Everyone brings their best dish that is from that culture. Spaghetti, lasagna, Alfredo, all kinds of dishes. Maybe a sign up sheet so everyone can see what is coming and not bring the same thing.
If they're snooty have a professional singer come and do some lovely music in Italian. If they are a fun group have some movie or something that the stars go to Italy, or race in Italy, or some other thing similar. Maybe someone got married there or has taken a wonderful vacation and can show 15-20 minutes of pictures. Not long though.
Chili cookoff:
Everyone brings their crockpot full of chili. Hot ones go on one table, medium on a different one, mild on another one. Perhaps the table clothes can signify the heat. Red=Hot!!!, yellow or orange=medium, and green for go=mild. Someone can bring hot dogs for the kids, Frito's for chili pie, cheese, sour cream, onions, chips, etc...lots of sides for this one.
Have the judges be sure they can handle hot hot hot hot. My FIL uses those Habarnero's in his chili. He almost always wins the hot category. Then some will make white chili, some will make a hot dog sauce style, some won't like chili and will maybe make some stew.
It's all in fun and for becoming a cohesive group, making relationships, etc...