Many hidden gems to discover but you need local insights
Singapore is a very strange city. I have visited there 4 times so far and each time I feel a little different about the city. On first sight everything is clean, tidy, beautiful but soon after I discover most of the major sights I felt bored. However there are very interesting neighborhoods that awaits to be discovered. I find the more 'local' 'authentic' side of Singapore much more interesting than the flashy touristy side.
Insider Tip:Type 'Hidden gems of Singapore' or Off the beaten path Singapore' on google and you are bound to find lots of interesting places. My favorite is Mac Ritchie Reservoir, Pulau Ubin Island. Pretty amazed by how many free things you could do/ see in Singapore. Hawker centres are also very good cheap places to experience local culture and food.