get some fresh air

“Fresh air” means the air outside. The air inside of a building can become “stale” and unpleasant, so it’s nice to go outside where the air is often cool and refreshing. You probably wouldn’t use this expression if the outside climate is very hot and humid, because that isn’t very refreshing.

We need some fresh air in here.

I think some fresh air will make me feel better.

To "get some fresh air" means to open a window or go outside, so that you can breathe air that comes from outdoors.

Turn off the computer, go outside, get some fresh air, and relax.

Let's (do something).

The phrase "Let's ___" is well-known to English learners. However, learners often over-use "Let's ___" and use it in situations where it isn't appropriate.

When can you say "Let's ___"? You usually use it when you're suggesting something that you're sure the listener will accept. If you're meeting a friend for lunch, then you're sure that they will agree to eat with you. So you can say:

I'm starving! Let's eat.

In situations where you're not sure how the listener will respond to your suggestion, you can use other phrases like "Would you like to ___?", "Why don't we ___?" or "We should ___":

Would you like to go out to dinner with me some time?

Why don't we meet at 7:30?

We should get together some time and have a drink.

in here

You can add “in” before “here” to emphasize that you are inside.

It’s hot in here!

cooped up

“Cooped” means kept indoors, often in a small space. (A “coop,” in fact, is where chickens are kept on a farm.)

“Cooped up” is very similar, and we use it to talk about people being inside too much, especially if they are inside for a long time. Being “cooped up” is not a good thing; no one wants to be stuck inside.

Winter is the worst because we have to stay cooped up inside so much.

It’s not fair to keep him cooped up in my little apartment all day.