Making the long story short, I've made many a friend with housekeeping staff by changing the USA coins they can't use into pesos.
It started a couple years back when I noticed a tray full of USA quarters on her cart. They were arranged in a pattern as if art. I asked her what it was and she told me. I responded it was money. Alas, it's not possible to change coins (moneda) into paper currency anywhere.
Now I'm the money changer. Last trip I came back with something like 12 pounds of coins!! The staff are always great whether I change or not, but now I always do and it seems like the number of people I'm doing it for is increasing.....all hotel staff.
I use the XE.com exchange rate which is essentially the LIBOR rate and is quite friendly to the recipient. It's an easy thing to do and now I've more friends.
When "tipping".....foreign coins are useless. Gotta be paper. The "group" stand around as we count the coins and we essentially have quite a bit of merriment as we go over what coins are from where/what nation. I'm think we're up to 32 different countries at this time. Anyway......it's fun doing simple things. One doesn't have to spend money or drink ethanol to have fun on Cozumel.