The Merlion park is not exactly very big for u to spend an entire day there. u can do Sunrise shoot there in the morning.
then stroll down to Marina Bay area. Currently there is the Orchids Conference going on, u might want to go for that this wk end at the Marina Bay Sands(MBS). That would take the whole day
You can enter the Casino at MBS, or u can pay to get to the top of MBS, u can try shoot Sun setting or the Marina Bay night scape.