Idle time means idle money. Think about how you would react if you were hiring someone and that someone had "idle" time while you're paying him. If you have investors, they expect you to do something good with your time that ultimately multiplies their money. If you don't have investors, you are your own investor (your time is your cost of opportunity) and you probably also want to make your time worth.

I've been in that situation and even while it doesn't feel ideal (just the fact of having idle time feels weird and wrong), I think its the best time to learn new things relevant for your business. There will always be something helpful to learn that can be useful for your business, so just try to prioritize on what would be the most useful in the short term. There is indeed many times idle time, but it is only idle if you keep it that way.

The easiest learning comes from reading. Keep up to date with industry trends, read about marketing strategies, read about cases of successful and failed companies similar to yours.

Learn to fulfill positions that are yet non-existent on your company. If you are a small startup and you have no one doing marketing, pr or something else, use your time to learn about it and do it yourself.

just my thoughts from someone similar