Eel River Brewry in Fortuna has good food, if you will be around there.
Fern Canyon is quite a drive from Eureka, but is a must see. You might need a four wheel drive to get there this time of year though. The dirt road out there can get really muddy, and it is steep right at the beginning. Your likely to see lots of elk out there.
Patricks point State Park is also a great place to hike around. It is just north of Trinidad. Great views, good beaches for walking on, nice camp sites (especially the agate beach area) and lots of trails. The ups and downs can get to you though if you aren't up to it. Hiking down to the beach or tidepools is great, but the climb back up can really hurt!
If I was going to Samoa cookhouse, the I would try to go for breakfast! That is always great!
I like the seafood at Gill's by the bay - especially the chowder. We like to get chowder, calamari strips and fries there. My dad says that they also serve up a good seafood breakfast, though I have not tried that. It is in King Salmon just south of Eureka, at the end of King Salmon Ave.
Across from the mall in Eureka, up on the hill, is the old fort. They have some old logging trains and other steam donkeys that are neat. They only fire them up once in a while, but they are neat to see. Lots of history in that park. We took the kids last summer and got to ride the train, see one of the steam donkeys run, etc.
Loleta cheese factory is kind of a fun place too. The kids like that. You can sometimes see them make cheese, and you can sample a lot. It is in downtown Loleta (between Eureka and Fortuna). Ferndale is neat too. Lots of history there.
The old Scotia Mill used to have a walkthrough self-guided tour. They went bankrupt, got bought out, and I am not sure if they still have anything there, but seeing that water powered debarker was sure amazing! It would grab monster logs, throw them around like match sticks, start spinning them and blow the bark right off with a jet of water! Scotia also has a Logging museum in the town.
Fortuna has a small museum in the park that is open a couple days a week. I don't remember which days. It's the old train station. Not a lot in there, but several old small engines if I remember correctly. Could be interesting. I mostly remember that they had an old rototiller that my dad was interested in. He said that his dad sold them when he was a kid. The tines were thin and springy like a pitchfork. I had never seen anything like it.
If you head down to the Fort Bragg area (long drive), you can ride the Skunk train. That's lots of fun too.
Anyway, I am sure there is lots else to do too, and I hope you have fun.