Watership Down? I read it and loved it at about 9, but I know some of my friends read it at 14 and didn't understand it at all. Also as it's a big thick book it'd only be suitable if she's a bookworm already...
Anyway, it's a story about some rabbits who have to leave their warren and it follows them as they trek across country (meeting foxes and traps) trying to find a new home.
I also remember reading a bok called "Fox's Feud" and the accompanying series, but given that it involves a fox being friends with rabbits and moles and toads it is a tad unrealistic...
Or you could try "Swallows and Amazons", not actually about nature, but it is about a group of kids camping in a wood on an island in a lake. In homemade tents. Again, big thick book warning.
I also remember being fascinated by books of weird aand exotic animals. I had a book with all sorts of animals in, birds of paradise, how bird's nest soup was made, tiny 3-inch octopii with fatal poison, lionfish, sting rays, angler fish, platypii, giant butterflies and moths, giant centipedes and millipedes, and why harmless animals look like dangerous animals (like coral snakes and that other snake), watersnakes, flying squirrels, aye-ayes, ostrich eggs being so strong you can stand on them, and some small rodent so strong you can stand on it and it would be fine. And shrews that have to eat all the time just to stay alive. Just go into bookshops and flick through a few books.
I also remember having a book with easily identified trees, plants, seeds, animals tracks and flowers.
Oh, or you could get her a flower press, or a book that includes how to press flowers, and ideas of what to do with them afterwards. (ie notepaper, brooches, hair clips, own flower identification book)
A bird-feeder? I got one for Christmas at about 10, and me and Dad put it up right outside one of the lounge windows. After a while we got loads of birds coming to it.
Sqirrels couldn't get to it either, because they couldn't climb the wall.
Can't think of any more right now.