An exercise apparatus includes a platform for use in exercises in an unstable configuration. The platform includes a stepping deck for receiving a user; an arched base for rocking support of the stepping deck on a workout surface; and a resistance band assembly housed within the platform, with the resistance band assembly including a plurality of resistance bands. The exercise apparatus may include a frame that is adapted to removably receive and support the platform above a workout surface in a stable configuration for use in exercises. In another exercise apparatus, the platform is secured to the frame such that, in a first orientation, the platform is stable; and in a second, inverted orientation, the platform is unstable. The platform is plank-shaped, and the stepping deck includes two stepping surfaces, each surface being located on an opposite side of the stepping deck.