A good sandbox can be hard to find, but well worth the effort. Most that I see are too small for siblings to share easily, even if it’s just two kids. I prefer sandboxes big enough to allow a few kids to play, so they can share with friends.
Once you have the right sandbox, provide a selection of toys and some good quality sand. Make sure that it’s play sand. There are places that sell colored sand, but I stick with the regular stuff. Add a little water (but not too much) and the plain stuff will do just fine.
Whatever sandbox you get, make sure you can put a lid over it. Neighborhood cats love a nice sandbox, and that’s no fun to clean up. It’s even worse if you don’t catch it until the kids have been playing in it for a while.