Probably the last person you should listen to on this, I have like 20 projects that I'm not doing at all times and always ready to jump ship, but...

I keep a spreadsheet of all the things I'm interested in and would like to do and when I get lots of spare time suddenly, I consult the sheet to remind myself of something that I could do. Most of the time none of them feel good, but often enough one does and I do that.