If camping or RVing is ok, Lake Del Valle Park in Livermore is cheap and easy to book ahead of time:
There are cabins available, and hotels reasonably close by for those who don’t want to camp.
They allow campfires, so we have jam-spots – and the campsites are far enough apart that as long as the playing is acoustic, we’ll be fine.
I’ve done jams during the annual Two-Day Town music festival there, and it’s worked well.
It’s usually hot during the day and cools down quite a lot at night, so it’s comfortable for sleeping.