This is a 3.6 wide deck. Suitable for a private home, but not for lots of kids, mums and activity.  It’s too narrow.

Recommend 4.8 wide. This means two 2.4 sheets of 19mm H3.2 plywood for every 1.2m length. No cutting plywood.

4.8 also is the size of the 100×50 H4 rough sawn timber that sells for $27 each which may be the most cost effective if the posts are donated.

Then either use non-toxic deck paint on the plywood, or look for a surface treatment that can be donated tile, poured concrete (like Mexican tiles) or whatever you can find. The plywood is the structure.  In this photo we used marble tiles because we paid $3/m² for the tile. Under it we used exterior 19mm tile underlay from TradeMe, $45 each over the existing structural deck.

4.8m wide makes it an outdoor space, a platform for activity as opposed to sitting still. Not all has to have a roof over it, just some shelter from rain and hot summer sun.