The Gift Shop at Muir Woods National Monument is actually pretty neat. They have a very large (and VERY expensive) selection of redwood burl crafts - from clocks to boxes and any other souvenier you'd normally find not made of wood. It's fun to browse, but, as I said, it's really pricey. I can't offer you an alternative - it's not like Redwoods NP where there are artisans in the immediate area. If you're interested in redwood burl furniture, I'd take a look in the Marin yellow pages, but, to be honest, I think you'd have to go out of the Bay Area to find anything not extremely overpriced.
What to buy: I would be happy for a letter opener myself. My birthday is in May.
There is a good gift shop near the entrance of Muir Woods. It's a good place to pick up some nice souvenirs or memorabilia. Goods purchased here go toward a good cause - preservation of Muir Woods.