Hey Kevin! I'm from Singapore so maybe I can advise you a little better.
Currently the weather is very, very humid and hot. A little haze is also present around here due to the burning of crops over in Indonesia (the quickest / cheapest way of clearing farms). What this means is that carrying a lot of gear or bulky / very padded is not at all advised. You will tire from it unless the main reason for your visitation is photography.
Are you sure all you want to carry is camera gear? You might want to carry stuff like a small umbrella / poncho and a water bottle for hydration. Like you said, weather is unpredictable, and it might be all hot and humid one moment and then the clouds might open up on you the next.
I'm a Think Tank fan, so I'm gonna say get the UD series. A UD40 might seem a little too big for what you have right now, but you might want to get at least one more extra lens in the future, and that's going to be perfect for the UD40. Has a dedicated space for magazines (think travel maps, etc) and little pockets for all the trinkets you might want to get. I recommend that you have a small umbrella in the unused lens slot, and a small water bottle outside. It might rain and you still might want to be shooting one handed. The UD40 can be attached easily to your luggage bag handle (assuming you own a bag with two telescoping beams) and can easily be locked and mini-bicycle chained to some pole. I wouldn't recommend that, though. Police do get jumpy around here with black bulky bags and no one around.
What about a tripod? If you're going to be shooting at night, there are some great cityscapes to shoot such as the renowned Esplanade Theatres on the bay and the middle-sized Merlion (both of which are practically right next to each other), the new-ish Singapore Flyer, the lights of Clark Quay etc etc. I highly recommend that you bring a travel tripod if you can get one. For your lightweight gear, a 0 series benro travel angel is more than adequate. It's your choice, though.
Of course, the one thing you will take lots of photos of would probably be the food that you eat. Round any street corner is an eatery that caters to all palettes.
Hope you enjoy your trip to Singapore!
Cheers,
Zexun