We are quite limited for space in our outdoor area/backyard, so I built a sandpit under our existing deck for our toddler to play in. The cover is made from the decking wood and comes off in 3 pieces to minimise the weight of each. Has been a hit with our little one!
Steps
Step 1
Measured out the area we wanted the sandpit to be - ended up being roughly 1.2m × 1.2m
Unassembled the deck in that area
Used circular saw to cut the deck into the appropriately sized pieces
Reinstalled the deck outside of where the sandpit was to go
Step 2
Dug out a hole under the deck deep enough to allow 200mm high sleepers to be dropped in to make the frame
Step 3
Built the sandpit frame out of H4 sienna micropro treated pine sleepers and merbau decking panels screwed into the two end sleepers for the base
Dropped the pre assembled frame into the hole 
Step 4
Lined the inside of the frame with landscaping fabric to allow water to drain out into the ground around the sandpit whilst not losing all of the sand
Filled the sandpit with sand!
Step 5
Using the pre cut deck pieces from step 1, I made a cover for the sandpit in 3 pieces so they weren't too heavy. I attached a small chain to the middle piece to allow easy lifting
Step 6
All done! The sandpit cover can be taken off easily when we want to use it and covered up when we want more space on the deck. 
