Mariner will be quite a ways from downtown Singapore. The cruise ship and commercial port is @ 25 minutes from downtown by cab. Make sure you do the Night Safari at the zoo. Also, try lunch or dinner at Equinox. It's on the 70th floor of one of the hotels(the round one) downtown. Great views of the city. Take the cable ride to Sentosa as well, but I wouldn't bother touring Sentosa, because it's pretty much a tourist trap. Singapore is a very clean and interesting city. We were at Raffles, but the hotels around Orchard Rd looked very nice. Lots of shopping there, but prices are pretty high.