Most of the electronic equipment found in Rio is imported, so you won't find any big bargains. But then again you may need rechargeable batteries, and extra memory card for your digital camera, or even a disposable camera to take to the beach, or something.
And of course, if you use a traditional camera you may want to develop those photos to make sure they turned out any good while you still have a chance to take fresh ones in case they didn't. There's many one-hour photos and photo shops that will deliver your paper copies on the same day. You can also order prints of your favorite digital photos, and even try to negotiate discounts if you have a whole batch to print!