Italian Wino, I know you've asked about walk in's, but if your in St Helena I'd make two more appointments:
Randy Sloan - Match Vineyards
Bob Fanucci - Charter Oak
Randy is a great host and if he's in town he'll be flexible around your schedule and usually it's just you and him on the tour.
Charter Oak is located in a basement at the end of Charter Oak St in St Helena, across from the Napa Olive Oil Company you can probably walk there from anywhere in St Helena in 5 minutes or less. Bob works as an attorney full time, so he likes to make his appointments for right after work at 5PM.
That tour and tasting is as old style Napa as you'll ever get and the wines are pretty good too.