I didn't actually get around to posting this until September, but I'm backdating it. In mid-July, Cristin and I went and tried her first backpacking trip. My goal was to find a route that would be both relatively easy, but also fairly epic. I wanted to show her that backpacking gave you access to areas that you wouldn't necessarily otherwise get a chance to see, but I didn't want to make the trip terribly difficult.
I had backpacked in this area once before and knew roughly what to expect, so my selected route was to hike in from the separate northern (Butte Lake) entrance to Lassen Park, down past the Cinder Cone Volcano, over the Fantastic Lava Beds, camp and swim alongside the crystal clear Snag Lake (poorly named), and then back again alongside Butte Lake the whole way.
The trip went very well. The Cinder Cone was the only real elevation, and while it's quite steep and difficult (loose scree), it's only about 800 ft of climbing and it's optional. You can walk around the base of it instead very easily. The route is ~6 miles the first day and I think around 8 or so the second (ignore the mileage that MyTracks returns as usual). Below is a quick map of the trail we took. The blue line cut out just a little before returning to the start point because my phone ran out of juice, but the trail follows along Butte Lake all the way around.
View Lassen: Cinder Cone and Snag Lake in a larger map
I'd recommend clicking through to the map and viewing it in Google Earth. The most interesting features are around the halfway point on Day 1 (red line): namely the Cinder Cone and Fantastic Lava Beds. Cristin was also particularly excited about being able to clean off some via a swim in Snag Lake, although some fishermen did report seeing some leeches, so do be careful. Also, the eastern side of Snag Lake was much wetter, a little mud, but mostly this caused us some mosquito problems on day 2. Very minimal mosquitos at our camp though, not a big deal.
Anyway, if you are looking for a good easy backpacking trip that is certainly Epic, I'd definitely recommend this one.