[Detrus]

Probably say you're working on projects that do this or that, don't need to say it's a startup, could be an open source project, who knows.

[wccrawford]

Agreed. Working on projects on your own time isn't threatening, and show initiative.

Working on a startup on your own time says you're a short-timer and they shouldn't hire you.

[luminary]

This is a very good alternative.