I love the sandbox. My three children will literally sit and play in it together for at least a solid hour at a time. The scoop, dump, build castles, and the 15 month old sometimes decides to taste the sand (gross). While they do that, I garden, read, and watch them have fun together.
The sandbox provides many learning opportunities for your little one. It provides sensory experiences as they play in it. They get to work on their fine motor skills as they dig. They can play little scientists as they explore the sand--just to name a few. And if you are feeling the need to get some extra learning in there, you can do sand letters or twist that a bit and do sand shapes. You could bury special treasures for your children to find. But don't feel like you need to do anything beyond what they come up with themselves.
If you don't have a sandbox but have the ability to get one, I highly recommend it! If you can't get one, see if you can play in a friend's sandbox or visit a park with a sandbox. Or you can always fill up a small, flat tote with sand to play in. So during these summer months, sit back and relax while your children learn away as they play in the sand!