We've added ducks and fish. The ducks were just cut out of construction paper and I let the kids put them together (with a little guidance so that the feet weren't on their heads). The fish were painted by their feet. I just painted the toes a different color and added a thumb print fin at the top.
We made cattails for the pond and they were messy but fun. I had my kids do them outside on the patio and then we just washed our hands with the hose. They each got one toilet paper roll and 1/2 a papertowel roll. It made one big and one little cattail. They rolled it in glue and then rolled it in sand. I cut out the leaves.
**hint: if you ever need pencil boxes like this go to the thrift store. I have gotten like 25 from there and I usually get a bag of about 6-10 for $1.50-$2.50. They come in handy for all kinds of things, but mostly for storing little games and stamp sets.