OP here. I really appreciate all your responses -- it has definitely given me some things to think about. As someone noted, providing an array of simple sides would not be difficult to do, and I am leaning that way. If anyone asks, I can encourage them to bring something, but I won't make it part of the invitation. Among our circle of friends/acquaintances, most are likely to show up with drinks in hand, at a minimum.
When we entertain, I generally set up food buffet-style in the kitchen, and people can bring their plates outside to the patio. That way the food isn't just sitting outside. I guess taking charge of the side dishes myself would afford me better control over the options, layout, keeping things cold, etc!
We've lived in this house for a couple of years, so no worries about people misinterpreting it as a housewarming!
I will definitely put an end time -- probably 12-4. I don't really mind if people hang around a little longer than that (truth be told, my in-laws are notorious for that anyway, but I just put them to work watching the kids while I clean up!
). With the open house idea, what I was trying to address was those people who might not want to stay the entire time -- I'm thinking of older neighbors who, say, would love to drop by and say hello, but aren't interested in swimming or staying out in the sun too long!