I think the answer may have been obvious to other people, but it wasn't until I spied a group doing this at Millennium Park that I realized how comforters can actually be the perfect picnic blanket:
A full-sized comforter will accommodate all four of us with room to lay and stretch out. Plus the extra padding can be super-comfortable:
It turns out Dwell Studio for Target even has some really cute options. If you watch the in-store sales you can usually get a really, really good discount. They have enough stuffing to be comfortable, but they're still thin enough to easily toss in the washer making them appropriate for daily use.
The only downfall is that it is too cumbersome to bring on our daily walk; we would have to plan to use it when we're not walking longer distances. Right now I have it rolled like a sleeping bag, but I wonder if I can find some way to keep it more compact?
But the big benefit is that we can enjoy having the pooches sitting on the blanket with us. It's just unfortunate it took me the whole summer to figure this out.