I like the scavenger hunt idea - which fits well with your theme. I have done this many times with different ages. One way that works well is to have a list in each ziploc bag and the items found must fit in the bag. The first to finish can choose a prize from a prize bucket - cute dollar store items (not wrapped) and then next to finish chooses next so that everyone gets a prize when they complete the hunt. Here is the list I made most recently...
Find 10 or more of the following items to win a prize:
something red, orange, yellow, green, blue, purple, other
items made of paper, plastic, metal, stone, other
If you have scoped out your area ahead of time you can list specifics like pinecone, feather, 4 leaf clover, flower, leaf, pebble, etc.
And when everyone has a bag full of "stuff" you can pull out the glue and cardboard and have the girls make a creation out of their items - a collage or a creature. But don't tell them this ahead of time... you'll get some great items to work with. At the very least you can take some pictures of their creations to send in thank you cards if the glue doesn't hold well! You could even have them make silent votes for the most creative - or have the lingering adults judge - to win a creativity prize.
Have fun!