I think that most people would agree that doing outdoor activities solo carries unique risks and can be quite different (there is also probably a portion of users whose default reaction would be don't).
As far as tags go, as of right now there is one solo-trekking but while the questions tagged with it all relate to being solo, not all relate to trekking, which is different from say kayaking or hiking.
There are also a few questions that have to do with being solo but are not tagged with solo-trekking.
After more thought, I don't like the idea of having a solo used along with other tags since if for example I saw solo and hiking used on a bunch of questions, I would think that we need to add solo-hiking
Here is what I am thinking now, create tags for each type of activity like
These would be examples of questions that I am thinking of,