The Night Safari is awesome and if you're on Sentosa stop by the luge and pink dolphin show (dolphin lagoon) :)

The night safari is cool, but if you are on your way back from a safari maybe it isn't the best use of your time.
Go to a really chill night at the beach at KM8 bar which is at the end of the Tanjong Beach Carpark on Sentosa Island. Very close to the city center and (especially if it is a sunday night) it is a fun atmosphere with live music and nice drinks. Another place that not many non-locals know about is Blu Jazz in the Arab Street area. (Near Khandahar street.) Great music and you can go beforehand for dinner to one of the arabic places for dinner...sit on the floor, enjoy the food and see a bit more of the local life.
Another favorite of mine is to enjoy a bottle of wine in the jungle/garden of a place called the Wine Company which is at Dempsey Road. (Dempsey Road is an area of town with lots of restaurants which tourist don't really know about.)
A beautiful lunch spot (if you don't mind putting on nice clothes and spending S$50 each) is to go to Au Jardin in the botanic gardens. (You must reserve.) It is a beautiful restaurant with impeccable service and is in a colonial black and white (traditional colonial style) house located in the Botanic gardens.

oh and nice hotels:
Lush island / tropics with amazing spa: Sentosa Spa and Resort (about 15 minute / S$9 taxi ride from the city center)
Center of city modern and beautiful: The Fullerton or Raffles the Plaza
Quirky, high class boutique hotel: The Scarlet (near Club Street / Chinatown.)
Shopping / Orchard Road Central: Grand Hyatt