Be prepared for the heat - Singapore is hot and humid. It's an average of 32 degrees celsius 365 days of the year. Singapore is a very safe and clean city, and the people are very respectful. However, there's not too much to do. Probably don't need more than a day or two here. Also, if you're going to other beachy places (Phuket, Kota Kinabalu, etc), don't go to Sentosa. You will be disappointed.
Insider Tip:Alcohol and cigarettes are extremely expensive here! Buy duty free in the airport upon arrival. Note that if you're arriving by land, duty restrictions are much more heavily enforced and you're only allowed to have one pack of cigarettes - they will confiscate the rest.