Looks like a great hike.  Did you say it was an 4 hour hike with 1,000 feet in elevation?

What's the underground 'building' in photo #1

We picked a longer loop and it was 8 miles, and 1700' net climbing and we did it in 4 hours.  It was pretty strenuous.

There is a shorter loop.

You can also just start up the 'Sea to Summit' trail and turn around at any point, I would recommend this, that way you can control how long and high you hike.

The underground building is bathrooms!   Really blend into the landscape.

looks lovely!

It's a bit of a drive from Geyserville (about 75 min) but if you ever want a change on your next time at G-ville, you and the kids might like the ocean by the mouth of the Russian river.    A couple miles from here, right at the mouth of the river, there are usually scores of sea lions basking on the beach and there are great spots to view them.  There are also many great little beaches just south of the river.

I thought the underground 'building' was an old World War II structure  :-)