A small SaaS. Small enough that it can be run by one person, part-time.
Any SaaS is a lot of work, and if you're interested in starting a Micro-SaaS I think your first product should be something simpler, see this startup stepping stones for an example of a path to bring you into the Micro-SaaS world.
My book "Choose Your First Product" is available now.
It gives you 4 easy steps to find and validate a humble product idea.