we don't have a sandpit as such, just a patch of sand in the middle of the lawn. yes, the sand/dirt gets spread around but they play out there for HOURS sometimes. So worth it.
Buckets of water are also an excellent investment.
My kids love to dig and they love nature. Maybe a mini raised garden bed for them to tend and do as they like. My girls had a lettuce farm that turned into a caterpillar farm and they loved it in both states. A bird bath/ raised bird feeding platform/ nest for native birds they could observe (probably best somewhere over grass though so you don't get bird poo everywhere.
I'm going to ask my DH to make I'm not sure what you would call it but think shop counter/old style lemonade stall with a counter and rails up the side and a small roof/awning for outside so they can play shops and use it as a work surface for out door play dough/mud pie making. Do you know anyone handy? Maybe the cubby house would do for this.
My kids love drawing on our concrete with chalk and making hopscotch pattns even though they still don't get how to do hopscotch.
At dd's old kindy they had a plastic oven and a real (but dead) microwave plus lots of old baking tins/pans/sieves and the kids loved using them in the sand pit.
In summer an upside down sprinkler tied to your sail cloth could be fun, or one of those hoses with holes dotted along it attatched to the sail (water restriction permitting)
And the best thing-children's imagination!My girls were playing pirates the other day sticks were swords, the Eskis were treasure chests, the table a boat and the dogs lead was their rope for helping people to board their boat. Poor dog thought he was going for a walk but in fact he was the shark!
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