Limited but extensible scope

You can't undertake a massive project on the side, so the scope needs to be minimal. Once an initial version is live, it should be extensible incrementally. Adding many small features that add up to a great product. Since side projects are primarily done in the evenings and weekends, the available time is usually chunked into sessions of a couple of hours.

That's another reason to find something that can be built iteratively. Try to ship something every other session.