We were just in Jedediah Smith for the first time last week. It's an amazing place. Be sure to drive the Howland Hill Road. There aren't a ton of trails in the park. I'm sure any of them would be good. We had little kids and old people with us ;-), so we only walked through the Stout Grove and did a couple miles on the Millcreek Trail, and it was great. There's a part on Millcreek where the trail runs along a fallen redwood and other plants and trees have grown up to create a green tunnel. There are huge banana slugs everywhere, and we even found a rough skinned newt on the trail. I had never been in a forest like that, and I couldn't get enough. It's amazing to be around such ancient living things and in a place that can support that much life.
There's a nice little sandy beach and rope swing right down from the bridge over the south fork of the Smith River. It was pretty fun to take a dip in the river.
After going there, I'm planning on reading "The Wild Trees" about a search to find titans that are hidden in the forest. It would probably be cool to read it before spending time there.