Up one way, down the other.
Much will rest on your budget and how much time you have available for the trip and of course, how long your budget can keep you on the road. There is a huge difference, both in time and distance between going to the "top of California" and Canada.
Rather than going "up and down the coast" I would suggest looking for an inland route 'up' and a coastal one 'down' and visit different places. For example you could visit Sequoia/Kings canyon and Yosemite, Lake Tahoe, Lassen, Mt Shasta and across to the coast to Redwoods NP and then take the coast home.
As Lifey said, plenty of time for planning, but to start with you should all sit down and discuss what you all want from the trip and that each one can match the time and budget required and you are all thinking from the same page.