I found a recipe for sand dough this evening, and it looks like fun. I wonder how much some play sand would be? Monkey loves this sort of thing. He'd have a blast mixing up the dough with me, and then just as much fun making stuff out of it.
The same site has a fun coral reef diorama idea that would be a fun project to do with Monkey. They actually have several diorama ideas. This polar bear habitat looks like fun. I'm in the mood for some sort of project like this, and Monkey's usually game for just about any art project I've ever tried with him. It'd be easy to turn something like that into a science project or unit study.