One of the downfalls of grappling-based martial arts is that you can’t practice a whole lot when you’re alone. With karate or muay thai, you can always shadowbox or hit a bag. You don’t have that luxury in BJJ; after all, you can’t exactly practice an armbar by yourself.
There are, however, a few drills you can do solo. Two good ones that come to mind are bridges and sit-outs. These two exercises simulate the basic movements you’ll be doing in a grappling match. They also make great conditioning movements. In fact, I would go as far as to recommend them even for non-BJJ practitioners. They build stamina, flexibility, and core strength.