Some brass instruments are ambidextrous as they don't have valves or any other movable parts, the hand is just used to hold the thing (e.g. simple horns, alphorns, fanfares, Bach trompets...).
Sometimes though the form of these instruments makes it easier to hold it with one hand than with the other...

A triangle should be ambidextrous, also most of the other rhythm instruments.

Xylophones (and similar instruments) I have seen with the low notes on either the right or left side.

Richard, I have seen 2 pianos (what is the English name of a "Konzertflügel"? Grand Piano?) built the other way round...
But they are rare and very expensive.
I guess as most piano players can't afford them when they start playing, they will stick to the usual pianos when they could afford to buy a left-handed piano as it is assumably quite hard to change from right-handed to left-handed once you are used to playing a right-handed one...

MudGuard/Andy