A couch potato is a person who sits on a sofa watching TV all day.
Here is an example:
John: Hey Jim, how have you been?
Jim: Lately, I`ve become a couch potato at home. I can`t stop watching that new TV drama! I should start going to the gym.
John: The gym [...]
In this idiom, there are two meanings.
1) You are thinking about doing something you need to do later.
2) You are always thinking about something you want to do in the future.
Here`s an example of each.
“I must email Maki about the dinner next week, I will keep that in the back [...]