I keep reading posts where some old people think that it is "cool" to tip with strange currecny, such as a $2 bill, or a gold coin with $1, or a canadian $2 coin (tooner)>
I can assure you that the people with whom I have spoken on cruise ships find those type of tips to be a pain in the butt. They get excited at first because they think it is a $20 bill, and then get depressed to see it is just $2.
As for the fancy coins, these are a royal pain, because nobody wants them. You can't insert them into even a coke machine.
Stick with US dollar bills. (ones, 5's and 10's)