"But thank you for understanding what it is like having a toddler and a dramatic 6 year old."
I don't understand this. You say "dramatic" and "6 year old" like they're different things...Seriously, though, JoyInVirginia has the same idea that my wife and I had, and it works out well. It allows for some spur-of-the-moment stuff without being too much trouble. In addition to the beach bag (our "bag" is a plastic bin with a lid that holds all of the toys, suits, towels, snacks and safety stuff/beach gear), we also have a park bag (that one is sunscreen, bug spray, extra socks and shorts, spare undies for both kids, first aid kit and trail mix type snacks so it suffices for any day trip stuff like hikes as well as the playground) so if someone invites us to go with them at the last minute it's not a real bother. It doesn't cost much in time or duplicate supplies and it makes "grab and go" easier. I understand that you're trying as much to get a good response so you don't have to go as looking for making it easier, but if you decide that you're comfortable with setting compromise farther to her favor than your own, then I figure this might make it easier. For myself, I tend to like planning but over the last few years I've definitely done more stuff at the last minute since our circle of friends are all "growing out of their diapers" and so the families are able to do stuff like this much more often.