Ask HN: What do you do with your time in between projects?

All the things you mention are good options. I spent a lot of years traveling and rock climbing in between contract gigs (though I tended to maximize the time between those gigs on purpose).

I built both of my day-job-replacing products in between gigs as well.

Don't sweat the short time period. The stuff you build now will still remain built even if you don't touch it for another six months. (Be sure to pick a tech stack where that is in fact the case!) Chip away at it when you can spare time from the day job and get it running (and charging money) in the background as early as possible. SaaS often takes several years to spin up into something to replace your job (mine took upwards of 10 years), so the earlier you launch the better.

I wrote an article about this not too long ago:

http://www.expatsoftware.com/Articles/guy-on-the-beach-with-...