Nice! After playing with it for a bit too long, this was my favourite:
sin(time * 0.2) * cos(time * 0.1) * (125+125*sin(0.01*(2 + sin(0.6*time)*cos(0.4*time))*((- 50 + x + 20 * cos(0.5*time))^2 + (- 50 + y + 20 * sin(0.3*time))^2)))