Matting can get uncomfortable hot in summer and fine bark gets into shoes and clothes and then ends up all over everywhere. I'd stick to grass.

A cover for the sandpit is a must have or it will end up being a toilet for every cat on the block.

In summer some kind of water play is lots of fun and cools everyone down. There are water play tables you can buy or you could just make your own with a bit of imagination and a few tubs, buckets and bottles. My boys are happy to spend hours moving a few buckets of water about the place and in the evening you can pour it out on what ever part of the garden needs it most.