Jujutsu unlike Judo, doesn't necessarily require a partner to practice.
The difference between JJJ (japanese jujutsu) and judo, is that JJJ incorporates strikes* So yes, you will have something to train on during your "off-times".
Jujutsu unlike Judo is more of a 'study-art' which requires more memorizing, more understanding, more analyzing, more critical thinking, more theory work.
The difference is that you'll be more versatile in the long term, you'll possess more technique, more knowledge, more versatility, more experience outside your comfort zone.
But beware, more doesn't necessarily mean better. I believe in using what little you know and make that work. Whereas JJJ practitioners are prone to be overflowed with information and will sometimes freak out when they can't decide what to use, Judo players will be more comfortable as they've done it a millions time before using this one technique they're comfortable and confident in.
Basically, if you were to try and explain them. Judo drills the simplest, effective and efficient techniques into your head, whereas JJJ you study a wider range techniques, analyze, categorize and work on them.
What can you train by yourself? Well you could practice striking and maybe go through some of the techniques you learnt in class.
In my honest opinion, I rather use the time outside of class to develop conditioning. Basically, in class you think "MA". Outside you think "Good Conditioning".
It doesn't matter what art you choose. It will be more or less like this. There are somethings you can work on at home, some you just can't. Usually, the most useful techniques can't be practiced alone.
I got two fists.. Don't make me use my head as well!