For the truly dedicated, and already strong, "campusing" can be one of the most effective power training techniques available for climbers.
Without a doubt, wooden rungs give the best overall surface for low-impact, (relatively) pain-free training.

Each rung is approximately 40cm wide
Each 5-pack of rungs comes with a comprehensive piece on campus board construction and training techniques.

Small: each rung is 19mm deep
Medium: each rung is 25mm deep
Large: each rung is 32mm deep