3 nights in Bombay is better than 2, but you have been there so you can take a better call. 5 nights in varkala + kovalam is quite good. I have stayed in Leela kovalam and it's really nice. It's atop a hill which is directly above the sea, with wonderful views. The drawback, the beach facing the property is really really tiny.
Haven't been to taj which was further down after a small village.