Boy are you going to get a lot of answers on this one!
The first thing I would suggest is think about your hobbies. Ones that YOU and not the rest of the family like. Then think of places that you either visited on vacation or want to go someday.
Design the place to look like your special vacation spot and work your hobby into this design. So when your in this place you feel that your on vacation doing the very thing you love to do.
An exercise room, yoga room or a retreat are what comes to my mind.
I recently made a Pinterest mood board for woman caves. check 'em out!!
A yoga space could also double as a meditation room. You just need a table for candles and maybe incense or flowers, and an iPod dock. In an ideal world, I would set up a meditation room with a perfectly programmed surround sound system and a big beanbag in the corner, so I could either meditate or just chill and listen to music.
What is your favorite thing to do? But don't have space for it? Crafting? repurposing? Painting? Singing? anyhing you love! he "woman cave" sounds grea!! Just all about you and your likes!
I enjoy reading and crafts...along with candles and wine ;-) Thanks for the responses.