the thing that Canadian two-dollar coins have -- a slug of one alloy inset in the ring made of another alloy..
The British £2 coin (introduced quite recently) is such a design, as is the new 2 Euro coin, and the old French 10 franc.
I also wish our paper money were color coded....blue, pink, green, gray, light brown, purple, etc. Would make it much easier to sort
Many Brits on their first exposure to U.S. money seem to find it odd that all bills are the same size and color. (British notes at present are £5: Blue/green, £10 Orange/brown, £20 purple, etc.)
I find the style of U.S. banknotes to be reassuringly traditional. They actually look and feel like "real money."