If a hiking partner doesn't materialize, you might find the short trail up Lassen to be pretty comfortable as a solo hike. It was re-built a few years ago and is well-graded, easy to follow, and pretty busy during the summer months. You're biggest challenge might be crowds, not solitude! Consider an early start to be ahead of the pack and for cooler climbing (there's no shade). Great view from the top!
I second the idea of a solo hike. Even if you are not comfortable with going solo, you will likely find yourself with plenty of company unless you either go very early or late in the day. And like the other guy suggested, Lassen is actually a very short and easy trail if you are accustomed to doing hikes like Adams.