Try pipe cleaners, spools,old pots, boxes, PVC piping (easy to get at Home Depot) would allow for water as well....you can ask them to cut pipes different lengths and just get a few joints....she could easily put them together and they could be used for pouring water in.
At that age, we 1/2 buried a large ceramic red pot on its side (so looked like a 1/2 cresent/dome shape above the ground) kind ofHobbit/fairy like. DDs then covered it in moss, leaves, sticks, pipe cleaners (flags), fake flowers, rocks.... it was a very very cute addition to our garden. At the end of the season, I simply removed the non-biodgradable stuff. The only thing I helped with was burying the pot and getting out all the stuff and being clear on what they could and could not use to decorate it.
You could even bury a smaller pot/dish to make a small pond and easily add a small pulley on a flower pike or a small lazy susan (often found in the garden sections to turn plants to get rotating sunlight) with decorations that could be spun around.
fish tank aquarium rocks are very pretty and come in all sorts of sizes/shapes/colors, you could add some of those too for very little $...(Target/Walmart,Petsmart)