Singapore Day trips

I was originally planning a 5 day trip in Singapore, but I rebooked my flight 2 days earlier to save a huge amount giving me 7 days in total, including a day for jet lag recovery/chilling out.

I was wondering what would be a good day trip destination to check out during this time. Is there a place that would have good snorkeling?

I am flying from Canada and will go to Seoul after Singapore.