I just stick an avocado half in a plastic container in the fridge. Stays green enough for days. If it gets brown, it's a thin layer that doesn't affect the flavor anyway and you can remove it without losing too much avocado if you don't like it.
Here's a secret hack to bring back the flavor of old avocado in the fridge for a few days: a half teaspoon of soy sauce and a few drops of hot sauce. You can thank me later.
