I found this idea on one of the crafty sites I visit and think it's fantastic. I'm making each of my kids one for Christmas.
I-Spy Tubes
Basically they're clear containers filled with white rice, which hides a variety of objects that appear by manipulating the container. I bought 1.5 quart rubbermaid containers, but you can go ultra cheap and use a clean 2-liter bottle. 1. Gather at least 20 small objects. You can use things from around the house, like a penny, a clothspin, etc. or go to the Dollar store and buy items like little bouncy balls, etc. 2. Add objects and white rice to container and glue lid on. 3. Write up a list of the items hidden in the I-Spy tube. Include list with a fancy pen for each child. They can check off each item as they find it, and find the objects again and again.
I think my kids will love these. I know you said you had lots of nieces and nephews to buy for.