Big Basin State Park: ?access from US 1
 

We are traveling the coast highway from Half Moon to Monterey and would LOVE to see some of the old growth redwoods which we understand are at Big Basin, on the way. We were wondering if we need to travel to the Park Headquarters or can we intersect an area of the Giant Redwoods on the road to Park Headquarters. We were estimating about an hour to take in the Gigantic-ness of the trees and the driving directions send us to the headquarters. Thoughts anyone?

I love Big Basin and often stop in there if in the SF area.
You have to be on Hwy 236 to see the trees. It is a relatively small area up in the hills.
You would have to head up from Half Moon Bay and drive along Skyline drive, or further down the coast from San Gregorio. Then you head south on Hwy 9 or back over to the coast south of Davenport. The roads heading up from the smaller towns are small and winding. You will not be traveling fast, more like 30-45 mph.
Once in the park, you could spend as little or as much time as you like on the trails. Also a good place for a picnic.

Henry Cowell State Park is closer to your route and has old-growth redwoods too. From Hwy 1, you take Hwy 9 from Santa Cruz to Felton - about 10 miles or so, maybe less.

http://www.parks.ca.gov/?page_id=546

Plan the route you take to get to Big Basin to avoid Highway 9. Highway 9 is closed 24 hours a day about 2 miles north of Santa Cruz until October due to construction.

Big Basin is really not a "one hour" stopover. It'll take you almost that amount of time just to get into the park. Big Basin is best enjoyed as an all-day hiking trip.

Henry Cowell is a good substitute, but that involves twisting and turning on the two lane roads in the Santa Cruz Mountains. It's probably your best alternative.

Otherwise, I'd suggest doing it very simply and just drive straight down Highway 1 from Monterey and go to Pfeiffer Big Sur where you can hike as much redwoods as you could possibly wish - one hour, a day, several days.