:shock:... I DO NOT PACK LIGHT - but in my favour most of our os trips are 6 weeks and several climate changes. I would light to try just HCO -....So give me as many hints and tips .... I am willing to give this a GO.
For a start, I do like KitKat's suggestion!
And to be going on with: I empathise and sympathise.
Most of my o/s travel is like yours, with multiple climate changes, added to the need for Business wear, evening wear and casual gear. I understand the challenge!
If you MUST do the carry-on only - and I HAVE done it but never easily, then a few basics:
1) Layers.
That won't be so important for Singapore but you will still need to contend with the ferocious aircon and the possibility of rain.
2) Pashminas and scarves.
Add warmth, when required, and dress up a plain block coloured outfit of basic skirt/trousers and top. A couple of fancy gauzy scarves will add to your basic black for a glamorous evening outfit, along with some glittery earrings.
3) Minimise colours.
Choose black/white/beige/navy as your theme and make sure everything matches. Reduces the need for multiple pairs of shoes, handbags etc.
4) Colourful costume jewellery.
Again, to "jazz up" a plain outfit.
5) Cotton beach wraps/sarongs/pareas (call them what you will).
Use them as an additional throw, a sun protector, a picnic rug, a bathrobe, a poolside cover up etc etc.
They weigh nothing, pack flat, and can be thrown into a day bag for any eventuality.
6) A "Day Bag".
Can be used as your "personal item" when boarding and will take a fair amount of weight, if you use a lightweight fold up type. If you want to take a special handbag, make sure it is lightweight, then pack it inside the day bag, along with your reading material, liquids and gels etc, so it won't be part of your carry on bag.
7) Most important: A champagne stopper.
Just in case you have only time for one glass before you head off to dinner! The rest of the bottle will be in good shape on your return.
Okay, I'll stop there.
Probably nothing you didn't know - and there may have been six great posts while I've been typing this one!
Best of luck.
pj