I tried to come up with a party favor that was practical but still fun. Since we didn't invite many kids, I had time to make robot shirts for all the kids. I used the freezer paper stencil method.
If you haven't tried it before, here are some quick directions. Draw a simple design or print one on the computer. Tape freezer paper to a cutting mat (freezer paper is parchment paper with a waxy plastic backing that sticks to fabric when you iron it, but peels off easily. You can usually find it by the aluminum foil at the grocery store). Tape your design onto the freezer paper and then use an exacto to cut out the places that you want color. Once it's all cut out, iron the freezer paper onto your fabric or t-shirt. Here's how mine looked.Put some paper underneath in case the paint bleeds through and then fill in your design with fabric paint (you can buy fabric paint at the craft store and it's pretty cheap). A lot of people just like to dap on the paint using a sponge brush, but I like using a paint brush and stroking from the edges in.
You will probably need at least two coats to get the color even. Let the paint dry between each coat or use a blow dryer to speed up the process. It should look something like this.
Then just let the paint dry before you peel off the freezer paper. If you use the metallic paint like the silver shirt, you might want to peel the paper off before it's totally dry because the metallic paint was thicker/rubbery so I think it might tear if you let it totally dry.