Design Your Own Doll Shirt
This was the most expensive American Girl doll activity I've done. We bought plain white t-shirts (at about $3 a shirt) and let patrons come decorate them however they liked. Sewing up an easy dress or skirt yourself might save some money, though, and give them more to decorate.
I put out lots of stamps and stencils to choose from that I already had. I had fabric markers and paint. I had some fabric paint, but I also got out my regular acrylic paint for these because, hey, they're doll shirts. I cut out squares of leftover chipboard and stuck inside the shirts.
I considered doing tie-dye, but where they're so small, I thought there wouldn't be much to it--they'd be finished in seconds. And I only wanted to do one shirt per attendee. But the markers and paint went over just fine--they seemed to enjoy it, and they had front, back, and sleeves to decorate. Would consider doing again.