I would suggest something flat and clear. You don't want it deep because then they dump them all out to find something at the bottom. You also want clear so they can see stuff to pick out (again with the dumping). I'm not sure at what age they can really keep stuff sorted - somewhere between 5-8, is what I have observed - so I wouldn't get a bunch of small compartments just yet, maybe a couple of small containers to go inside the bigger one. Something like the flat scrapbook cases at Michaels/JoAnn with a couple of smaller ones that fit inside?
Mama to three boys ('03, '05, '07)