Our summer plans include garden parties. Mom wants to have her friends over and I am going to have a "big" birthday. Her house is tiny and we have spent 6 years cleaning the lot from a bad episode of hoarders to almost a cute yard. It is designed to have "rooms"' so a variety of places to sit either under trees, by the coop etc.....
I set up a table with food, potluck style and plug in the crock pots. Makes lift easy and food stays hot. I don't think we have yellow jackets, but if you know your location will have them, I would start trapping them now.
I string Christmas Lights in the trees etc, we have a fire pit, marshmallows and etc. I have used canning jars for flowers, lemonade to drink, very casual. But for my "Jubilee" birthday, I am thinking of having the gals wear some funky large hats and decorate with flowers and the guys with black top hats (pretty cheap on amazon)
I would totally make sure it was a co-ed party, you can always suggest a small gift card to a couple of places, so you can pick out the right things for Ken (Costco, Bed Bath..Home Depot) that kind of stuff.