Many people choose to combine a visit to Singapore with a few days at a nearby beach or Island destination.  Here are a few places a few hours flight or ferry from Singapore -

Bintan Island - http://www.tripadvisor.com.au/Tourism...

Batam - http://www.tripadvisor.com.au/Tourism...

Tioman - http://www.tripadvisor.com.au/Tourism...

Nikoi Private Island - http://www.tripadvisor.com/Hotel_Revi...

Nikoi Island Website - http://www.nikoi.com/

Langkawi - http://www.tripadvisor.com.au/Tourism...

Boracay, Philipines - http://www.tripadvisor.com.au/Tourism...

Article on the most Pocket-friendly Beach Destinations in SE Asia -

 http://travel.asiaone.com/photo-galle...

Top 10 Beach Destinations -

 http://travel.asiaone.com/article/des...

Island Getaways on a Budget - 

 http://travel.asiaone.com/article/des...

Little-known by visitors - these places and Islands in the article below are accessible from Singapore (Pulau Ubin, Kusu Island, Sisters Island, St John's, Lazarus Island)

 http://travel.asiaone.com/article/des...

Wild Singapore - Pulau Ubin - http://www.wildsingapore.com/ubin/

Southern Islands - http://www.sentosa.com.sg/en/nature/s...

Lazarus Island - http://www.expatliving.sg/blogs/A-ste...