you should be able to do this in most major banks in Singapore. Go to the one that you have an account in, so as to avoid fees in converting currencies. Some bank will waive the fees depending on the nature of account. e.g., if you convert your fixed deposit in Singapore dollar into US dollar, they will most likely waive the fees.

I suggest request a bank draff in US dollar, which is just like a cheuqe you can deposit into any US bank when you are in the US. I wouldn't suggest cash or traveller's check, since they only have notes in $100.