Since you want to both stay at the beach and visit the redwoods, I think your best bet would be to head to Sonoma or Mendocino County.
When I looked up Pacifica, I found a few hotels at or near the beach (Holiday Inn Express, Sea Breeze Motel, and Best Western Plus Lighthouse Hotel). But in order to explore the redwoods, you'd have to go all the way south to Santa Cruz, or north to Muir Woods in Marin County.
However, if you head to Sonoma and Mendocino Counties, you'll be right in the thick of redwood country. There are also several parks there that feature redwoods: Russian Gulch State Park, Van Damme State Park, Navarro Redwoods State Park, Gualala Point Regional Park, Salt Point State Park, Kruse Rhododendron State Reserve, and Armstrong Redwoods State Park (located inland north of Guerneville).
Most of the hotels and inns there aren't actually on the beach so much as on bluffs overlooking the beach. Keep in mind that North Coast cities and towns are pretty small, so your options will still be limited. However, there are some very nice places available. Fort Bragg, just north of Mendocino, is the largest city in the region, so you'll find more variety there.
A lot of the places along this section of coast are bed and breakfasts or small inns, and they sometimes have minimum stay requirements on weekends. Just something to keep in mind.
I did a quick search to see what I could come up with, and I've listed a few options below. These are just a few selections, to give you an idea of what's available.
Hope this helps.