It depends on how big your cage is. My pigs love to change houses - they have a towel house, a cat basket, a cardbox house, a plastic house which I'd remove but they like it, a hammock to hide under, a big hayrack, two fiddlestick tunnels... and twenty times a day my pigs exchange places. If you have a big cage, why not give your pig some variety to play and hide in?
Water dishes are only bad if the pig uses them as a toilet, or if every fast movement fills it with bedding. If you have a clean, dry spot where to place it, and if your pig keeps his bottom out of it, it should be okay.
I don't know if pigs are interested in mirrors but if it's outside the cage, there shouldn't be a problem.