The Mare Island Preserve Has been open for a while. They were open one Saturday a month now it is open every weekend Fri, Sat, Sun, 9-5. Google Mare Island Preserve for Days and times and events going on. This is the Entrance if the gate is closed it's not open. Either way you can still get to this cache 24hrs a day
It's a fun place full of history Too!
You can hike, bike, picnic. (they have bikes you can use too small kiddie bikes even Just ask)
It is free to go in but they ask for donations.
They have tours into areas you normally can't get to, those are free also. And guess what? there is a bunch of Geocaches in here also For more info google Mare Island Preserve they wouldnt let me put the link here sorry 
The Sierra club has a hike to the South shore, Bird watching hikes, Slide Show once a day. And We hid a couple caches here also.
So come check it out.
