Difficult to utilise space - ideas appreciated!

[LETMO]

My home is built jutting out of the side of a soft sandstone cliff; with the top floor essentially the ground floor & two floors beneath it (B1 & B2).

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Directly under the ground floor is an outside space about 8m by 4m. I would like to utlise this for something, but am at a loss for ideas. It gets some sun in the morning (not much because there is a significant overhang. I have 2 daughters aged 7 & 8. Any suggestions welcome.

Here are some pictures.

 

[Thermo]

things that come to mind

storage, but tidier than it is!

turn it into a play area for the children. deck over part of it so they can ride the bikes etc up and down, little play house, and make it intro a fairy grotto with all that exposed rock.

adult area, nice deck, water feature running through the rocks at the back, sheltered etc

 

[LETMO]

Thanks for the ideas. I realized that the 'grotto' idea was not appropriate as my girls are growing up so fast, but that in turn made me realise that they are going to need some privacy down the line (our top floor living area is divided by paper shoji & fusuma). So I think I'm going to take some time & turn it into their bedrooms.

Big job! So I'll be back for more advice! Thanks again.