This was funny...

Have a nice day!
* Achcha din lo!

What's up?
*Uupar kya hai?

You're kidding!
*Tum bachcha bana rahe ho!

Don't kid me!
* Mera bachcha mat banaao!

Yo, baby! What's up?
* Beti Yo, uupar kya hai?

Cool man!
* Thandaa aadmi!

Don't mess with me, dude.
* Mere saath gandagee mat karo, ek husti.

Check this out, man!
* Iski chaanbeen karo, aadmi!

She's so fine!
* Woh itnee bedaag hai!

Listen buddy, that chick's mine, okay!?
* Suno dost, woh choozaa mera hai, theek?

Hey good looking; what's cooking?
* Arree haseenaa; kyaa pakaa rahee ho?

Are you nuts?
* Kya aap akhrot hain?

Son of a gun.
* Bachcha bandook ka.

And the best one is.....

How do you do?
* Kaise karte ho?