Sandpits - any recommendations please?

[zinfandel]

I am looking to get a sandpit big enough to last a few years for a 2 yr old and a 1 yr old but I am struggling to choose one. I'd be really grateful if anyone has any recommendations for which ones to choose or avoid. Thanks!

[Camelbak]

My friend in Holland had a brilliant one- was like a big clam shell so you could put sand in one half and use the other as a lid or paddling pool (obviously you don't want to make cement though). They had a tiny concrete backyard.