The clam shell ones are good because you can use a second shell to cover them and stop the local cats from visiting at night.
I'm in two minds about our sandpit. The boys love it - it keeps them entertained for hours but I hate it. I have to check it for cat poo which totally grosses me out and the sand gets everywhere which drives me crazy. It gets in their pockets and then falls out all over the floor and they dump the sand outside their pit while they play with it in the garden and if I'm looking the other way they get a dump truck full and drive it under the car-port to make a secondary mini-sand-pit.