how about a fabric ball with all sorts of fun patterns and textures. You could also make fabric blocks with the same idea. theres always the possibilty of making her some darling little clothes too (love ottobre for this). and some matching booties or hats!
I got some quarter flats of cotton quilting material from Joanns in all kinds of patterns and colours, then sewed up some quick shapes for my daughter. When she was teeny tiny she loved to stare at the black and white patterns, and later the colours caught her eye.
What about a little blanket for the doll, knitted or sewn? I remember my daughter loving to wrap or cover her "baby" in a blanket when she was just a baby herself.. .
Also, homemade wooden blocks, or homemade tree blocks? She would only need a few- just enough to stack.
I was also thinking about bean bags- but I guess a 6 month old could make a huge mess of them if she likes to suck on things!
The Waldorf doll book that I have suggests REALLY simple things for babies that age - a crocheted wool rattle stuffed with wool and a single jingle bell, a cloth ball (try to find a pattern for those old-fashioned scrap balls!), felted wool balls (I make those with scrap wool yarn. Wind it into a ball, felt it down. Wind on more, felt it down.), a simple knot doll.....
Our 6 months old loved boxes, smallish items to play with in the boxes (we had wicker baskets) (depends on your comfort level with choking hazards), we had pinecones, largeish rounded rocks, tiny playsilks, flat rounds of tree trunks we'd cut into little 'plates' (our kids like these even better than the branches), felt balls (easy to make) etc..
My little guy really liked what I called a Crinkle Bag
I made a sort of taggie pouch and filled it with crinkly mylar that I saved from packaging, he loved making it crinkle and messing with the little ribbon tags.
Here's a picture... http://static.flickr.com/56/162600387_01390f776f_o.jpg
I have the best luck with just using wrappers from cookie packages or anything that makes noise when you unwrap it.
You know the kind that makes the dog or cat come running to the kitchen. LOL!
You can also use the mylar that's on the roll you can get in the silk flower section at WM but at least with the wrappers you get to do some recycling and I gotta love that!!!