couple things-
bugs are great!
you could, tie them to a chair in a room full of botflies and let him chill out for a few weeks, giving him just enough water to stay alive. the starvation would suck bad enough, but bitflies lay eggs in human flesh when they get hungry enough, and the eggs hatch into maggots that will eat you from the inside out.... slow torture.
or, do like the apache-
stake a naked man to the ground over the biggest anthill you can find. preferably red ants or fire ants.
animals:
you could starve a rat for a week or 2, it will get good and mean, then use a 2.5" tube forced in the guys mouth to drop the trat in.... the rat will eat his fill, and the come back for more. there are several variations you can do with this.
an aggressive dog will fuck a human if given the chance.... whether u have dude tied up and bent over or dude tied up and his WIFE/GF bent over.. you get the idea.
electricity, when applied properly.
use batteries, its easier to control the direction of current and much more painful.
series a few car batteries together to get 60-70 volts of dc and:
but the electrodes across his nuts
or perhaps one electrode under the finger nail and another strapped to the wrist... burn the fingers and toes off one at a time!
you got to get creative with torture.
fuck a waterboard, that shit is for pussies