You don't say when your cruise will begin and end, so it's rather difficult to make recommendations. If you're looking for expanding your travel experience, I'd skip Singapore (city-state) and consider any other destination in South-East Asia. It also depends a great deal on what you wish to accomplish; rest or looking at the sites?
There are a limited number of activities you can accomplish in Singapore such as a) orchid botanical garden,

night safari, c) Sentosa Island, d) Raffle's Hotel Long Bar, and e) the many good restaurants along the river.
If you want more info on other destinations in that region, please let me know.
