Silvia Trkman's Cik & Cap training is great for teaching tight turns. It's very easy, step by step and teaches the dogs to find turning tightly to be very motivating.
Lots of off leash running, swimming, etc is always a good idea for general fitness and flexibility. For flexibility specifically, as well as balance - tricks are the best! Off the top of my head: handstand, lifting both side legs at the same time, spinning both directions, dancing on hind legs, weaving between legs (forwards and backwards), circling an object quickly in both directions (first step to Cik & Cap), putting all feet into increasing smaller and smaller boxes/bowls, getting into and laying down in smaller and smaller boxes.