The best kinds of side projects are the fun ones. When you really enjoy working on something, you really don't need to "stay motivated".
I love to code and I love to build things, and I would lose my sanity if there was only one thing I ever worked on. Plus, one of the best things about side projects is that there's usually no pressure to really get it done, so when I load my workspace for a side project, I'm always excited to get started, because I know nothing really important depends on it. And unsurprisingly, I've learned so much from random fun stuff on the side that I would never have learned otherwise.
I love to code and I love to build things, and I would lose my sanity if there was only one thing I ever worked on. Plus, one of the best things about side projects is that there's usually no pressure to really get it done, so when I load my workspace for a side project, I'm always excited to get started, because I know nothing really important depends on it. And unsurprisingly, I've learned so much from random fun stuff on the side that I would never have learned otherwise.