I like the Scrubbing Bubbles peoducts. They seem pretty mild yet remove soap scum from tubs and water marks and toothpaste from my nickel bathroom faucets.

I'm not sure how effective alcohol is on soap scum but for your travertine (or any natural stone) and nickle fixtures, you can't go wrong with the 50/50 "homebrew" of alcohol and water. It's my go to cleaner all around the kitchen and bathroom and with a microfiber cloth it's great for windows and mirrors too. The fact that it's so inexpensive is icing on the cake :) I learned about this solution on the Kitchen forum back in 2002. I don't think Scrubbing Bubbles (my favorite for soap scum) is safe for natural stone.
Monica

Do not use scrubbing bubbles on travertine. Besides, it doesn't clean well enough for me. I second the alcohol, water mixture.