I teach ballet and modern dance, so that keeps my body moving... I actually really recommend ballet as a good cross-training activity for cycling. It uses a lot of the same muscle groups (glutes, quads, hamstrings, calves, back) but also has some good counterbalances to cycling--the upright balletic posture can be a good counter to the cyclist hunch; balletic turnout of the legs balances the constant straight forward position of the legs cycling; jumps in ballet add an impact exercise component, good for bone health, that cycling lacks.

At any rate, I have found that the two complement each other quite well.

But if ballet isn't your thing, I also really like the exercises in Tom Danielson's book "Core Advantage"--the exercises in it focus on strength and stability while in motion, which really makes sense to me. We're never still on a bike, so we need solid core support that doesn't depend on holding still.