Since you leave Eureka, you are going to start entering the “wildest” part of California, you will pass through quite authentic towns. I recommend that you make a stop in Weaverville (this town is the capital of Trinity County and is surrounded by spectacular forests and mountains. It is well worth a walk along Main St -cross the town- where you will find several organic food stores, art galleries, a museum or the Joss House State Historic Park ), Lewiston (both on 299), and Lake Lewiston (West of Redding, a town that by the way, is absolutely expendable, in summer you rarely find something open at 10:00 beyond the gas stations).
Arriving in the Trinity Mount area is a true spectacle of nature, especially since this region is always crowned by Shasta Peak. Before reaching the National Park, visit Mount Shasta Village (the main streets are Shasta Blvd, Old McCloud Rd, and McCloud Av), and at the Visitor Info on Pine St, it is a small cabin with dozens of free brochures and magazines from the area and the State of California.
Mt Shasta is a region where you will be able to do a lot of water activities and trekking, so at least dedicate two-three days to enjoy its incredible nature.
When you arrive at the National Park (you can get there from the town by the I-5 N or by the A-10 Everitt Memorial Hwy), you will find different trailheads with their difficulty, in all of them you have to register beforehand so that the rangers have proof of the people who make routes through the park and in case you need some kind of assistance. The Bunny Flat route is not one of the simplest (it is all uphill), but it is very quiet, it is a little frequented route and the prize is at the top with a refuge where you can rest and refuel in a small spring. Respect the sustainability indications they have, no soaps, papers, and plastics, and the garbage that you generate is taken down again (it costs nothing;)).
Other easier trailheads are John Everitt Vista Point Trail (1/4 mile), Gray Butte Trail (2 miles) heading to Gray Butte, Panther Meadow Trail (it’s a 1-mile circle), South Gate Trail (2 miles), Sand Flat is the pre-pass to HorseCamp and Avalanche Gulch, and the highest trailhead, Sky Bowl Trail is primarily a route full of rocks and stones. Any of these routes will allow you to enjoy the second highest volcano in the United States, and if you catch a sunny day you will get some impressive photos.
When you leave the Park where the routes for Mt Shasta are, go down to the Siskiyou Lake area (you have to go by private car since I did not see any indication of public transport, take the Stage Road towards Castle Lake Scenic Drive Rd .), It is not very easy to find it if you do not have indications of the people of the place. When you arrive, it is a real luxury to swim in the lake and have the volcano in the background. It has areas where you can spend the day and also do routes on foot.
Another mandatory is Castle Lake, which is approximately 10 miles from Siskiyou Lake, in this lake, you will not be able to bathe but you can do canoeing or simply be resting on its shores.
Some of the towns that are in the area and where you can take a walk are Mount Shasta itself (it is a town with a lot of life, quite interesting shops, very lively restaurants and a natural environment of a movie), Dunsmuir (mainly the Dunsmuir and Sacramento avenues with several shops and restaurants) and McCloud (historic logging town at the foot of Mt Shasta, from here you can do several routes on foot). Their interest is mainly in the architecture of their houses, the tranquility that is breathed (you hardly see people in the streets), and having a coffee in some of the bars they have.
If you are staying in the town of Mount Shasta you can go up to Yreka (almost on the border with Oregon) on I-5 N (about 1 hour). This town has its origin in the gold rush, its historic center is quite cool to take a walk or rest in one of its parks.