We just finished making a wooden bike seat for my older son.
After making it, we decided we needed the back passenger and front passenger to have a little storage.
So the front passenger got an old plastic organizer from the garage screwed into his arm rest. It's just big enough for a banana and some nuts.
The back passenger got a storage bag that I made when I made the crate seat. We got a little crafty in attaching it, since it was made for handlebars. We found a plumbing piece at the local hardware store with wider ends (these keep the bag from sliding off). We also found two metal pieces to screw on each side. This way, the bag is still removable. Another option would have been to screw right through the bag. I decided against it because I didn't want to permanently damage the bag.