Playful/Affectionate:
* Nana
* Grandma
* Meemaw (common in the South)
* Grammy
* Gran
* Gammy
* Nanna
* Nonna (Italian)
* YiaYia (Greek)
* Abuela (Spanish)
Informal/Casual:
* Old lady
* Old timer
* Granola
* Gaga
* Granny pants (referring to underwear)
Slang terms can be quite regional or even family-specific, so the best way to know what someone prefers is to ask them!
It's important to be respectful and sensitive when using slang terms, as some people may find them offensive or disrespectful.