To kick back = to relax, unwind, rest

You can use this expression when you don't want to do something, but just relax and enjoy yourself.

For example: 
Dara: Hey! Vichea, Dyya wanna (do you want to) get out fishing with me tomorrow? 
Vichea: I'm afraid not, man. I had a long week and I just wanna kick back at home this weekend. You better call me next time. 

In retrospect = with hindsight, looking back to the past situation or experience.

For example: 
In retrospect, you'd better go 30 minutes earlier and be ready for your exam.
(you got flunked the exam last time and it was your bad experience.)

To sleep in = remain asleep, stay in bed later than usual
You use this expression to express the situation when you stay asleep and without an alarm clock or a hurry.

For example: 
I had a long week and need a rest for this weekend. I want to call it a day and sleep in the whole day.

Call it a day = stop doing something
When you call it a day, you stop doing something (working, studying, etc.)

For example:
Keo: Sam! We've been working on this project for 15 hours. Let's call it a day and have some shots of Tequila.
Sam: Okay! Deal!