Thers some great ideas here. I'm making all the children in our family I spy bags this year. To make them personal I have printed photo's of the family onto shrink plastic, so they can find mum or grandad as well as all the other little bits. I'm making these for my 12yo son and 16yo niece as well, as they both saw my prototype and asked for them.
Going to make them with a small window and the tiniest items I can find for the older ones to make them more difficult.
I've also seen people make I Spy bags for older kids with only all-white items in the white beads to make it harder.