Cash for 

• Restaurants - or stores! It's a simple way to avoid the 3-6% charge for using a credit card

• Small transactions like coffee, breakfast, and other snacks, bartering at the night and street markets.

• Entry fees to cenotes

• Taxis / ubers 

• Bars / drinking

ATM Options

There's quite a few banks that offer no international fees. You can set one up before heading out. Alternatively, my bank doesn't charge, but on-site, the ATMs I used charged from $2/transaction to $10 at the touristy spots. Easy way to save money and keep cash on you is to just get what you need for a few days and hit the ATM again. 

Some places will run out of cash, with the influx of tourists pulling it out. There was no cash in Tulum a couple times while I was there, you just need to wait til they refill it and try again. 

Avoid changing currency at the airport - they charge a lot. When you hit the ATMs you can pull out Mexican Pesos (MXN) at the current bank's exchange rate. Can't be said enough; when you get that screen asking if you want "XYZ Exchange Rate" ALWAYS decline it's usually marked up. I don't know why, I don't care. I know I want to keep my sheckles. 

Quick Conversions:

I usually pick a dollar amount ($10 or $20)--you choose what works for you. Then convert to pesos. Use something that scales up and down quickly for easy math. You won't be exactly on but you'll have a very good idea.

Example:
$10 is usually more than enough for a beer and food, it's easy to scale.

$10 USD when I was in Mexico was around 200 pesos. So, I just know $200MXN is = $10USD more or less. This helps with pricing things at markets, especially when your Spanish sucks (ragatear=to haggle/barter).

Food is a big one for me, so a lot of my meals were about $100MXN -- or? ... $5USD. This also scales up, but use whatever numbers you're comfortable with and use most often!