You can safely put Bentonite down your bath/drain IF you don't have a septic tank...just rinse the tub THOROUGHLY with lots of hot water immediately after you're done to ensure it doesn't build-up in the pipes.
If you have a septic tank, do NOT (EVER!) put Bentonite down the drain.
Activated charcoal in the bathtub? NOT a good idea. Well, it's good idea for detoxification, but it's likely to permanently stain your bathtub black (especially if there's any scratches in the enamel). And if it's not enamel, it could likely turn your bathtub black. Do it ONLY after you've tested it somewhere on the tub you don't mind ruining.
And don't forget - activated charcoal on any open wound or sore WILL get into your skin and permanently 'tatoo' your skin.