When solo, I spend a surprising lot of time in my tent, under my quilt, because I tend to head there when it's dark and getting cool. I don't carry loads of sit-in-camp warm gear so the quilt is king when I get chilly. Frankly I run out of things to do when the sun goes down. Sundown to sunup can be 12 hours, and I'm impatiently waiting for the sun to get up so I can have some fun.
Writing in my journal helps, but occupies maybe a half-hour. (And notebooks are freaking heavy, have you noticed?) What do you do with your solo downtime? I don't want to start bringing a Kindle, I really don't, but…I need more dark-time, down-time solo activities.

