Just ask the people you work with what their dream workplace would be like.
Then find a middle ground that is realistic and build a culture around it.
If the people you work with are the type of talent you want to attract then get inside their head and see what they like.
It really can be that simple.
Finally, live and breathe the culture. Don't create a place that sounds good on paper with no follow-through. Word of mouth spreads fast and reviews are easily accessible.